Bicasting using non-congruent paths in a loop-free routing topology having routing arcs

ABSTRACT

In one embodiment, a method comprises creating, in a computing network, a loop-free routing topology comprising a plurality of routing arcs for reaching a destination network node, each routing arc comprising a first network node as a first end of the routing arc, a second network node as a second end of the routing arc, and at least a third network node configured for routing any network traffic along the routing arc toward the destination node via any one of the first or second ends of the routing arc, the loop-free routing topology providing first and second non-congruent paths; and forwarding bicasting data, comprising a data packet in a first direction from a network node and a bicasted copy of the data packet in a second direction from the network node, concurrently to the destination node respectively via the first and second non-congruent paths.

TECHNICAL FIELD

The present disclosure generally relates to generating a loop-freerouting topology in a computing network, such as an Internet Protocol(IP) network.

BACKGROUND

This section describes approaches that could be employed, but are notnecessarily approaches that have been previously conceived or employed.Hence, unless explicitly specified otherwise, any approaches describedin this section are not prior art to the claims in this application, andany approaches described in this section are not admitted to be priorart by inclusion in this section.

Bicasting is employed in industrial wireless applications where bothreliability and timeliness of data traffic must be guaranteed. However,existing routing protocols are not well suited for bicastingapplications. Existing routing protocols assume recalculation of routescan be performed in response to a detected failure (e.g., loss of a linkor a connecting network node); however, recalculation of routes requirescomputation time that likely results in a loss of data traffic. Oneattempt to accelerate responding to a detected failure includes “LoopFree Alternates” (LFA), where a router can respond to losing a next hoprouter in a path to a destination node by identifying a feasiblesuccessor for reaching the destination node, as long as the feasiblesuccessor does not create a loop by sending the packet back to therouter. The IETF Draft by Atlas, entitled “U-turn Alternates for IP/LDPLocal Protection (draft-atlas-ip-local-protect-uturn-00.txt)”,illustrates in FIG. 3 of the IETF Draft a network topology where LFAsolutions can be provided for the network nodes N2, N4, and R3 toforward a data packet to the destination node D in the event of a failedlink, however, LFA cannot provide any solutions for any of the networknodes N3, S, P, R1, or R2. The IETF Draft by Atlas proposed a “U-turnprotocol” to reverse a link; however, the U-turn protocol cannot provideany solution for the nodes P, R1, or R2 of FIG. 3 in the event of a linkfailure.

Another proposed routing protocol, referred to as “Fast Local Reroutingfor Handling Transient Link Failures” (FIR) requires a completerecalculation of routes in response to a link failure, and hencerecalculation of routes requires computation time that likely results ina loss of data traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the attached drawings, wherein elements having thesame reference character designations represent like elements throughoutand wherein:

FIG. 1 illustrates an example loop-free routing topology comprising aplurality of routing arcs for reaching a destination network node,according to an example embodiment.

FIG. 2 illustrates an example apparatus for creating a loop-free routingtopology in a link layer network of network nodes coupled by data links,according to an example embodiment.

FIGS. 3A, 3B, 3C and 3D illustrate example methods for creating aloop-free routing topology, according to an example embodiment.

FIG. 4 (consisting of FIGS. 4A, 4B, 4C and 4D) illustrate an exampledata structure in the memory circuit of the apparatus of FIG. 2 used forcreation of a loop-free routing topology, according to an exampleembodiment.

FIG. 5 illustrates another example data structure in the memory circuitof the apparatus of FIG. 2 used for creation of a loop-free routingtopology, according to an example embodiment.

FIGS. 6A-6I illustrate an example sequence of constructing, from thelink layer network of FIG. 2, the loop-free routing topology having aplurality of paths (e.g., routing arcs), according to an exampleembodiment.

FIG. 7 illustrates the loop-free routing topology of FIG. 6I as anexample hierarchy of successively cascaded routing arcs, for propagationof a set of serialized representations of paths in the routing topology,according to an example embodiment.

FIG. 8 illustrates an example method summarizing the method ofgenerating and propagating a set of serialized representationsdescribing the loop-free routing topology, that enables network nodes toestablish loop-free label switched paths for reaching a destinationnetwork node, according to an example embodiment.

FIG. 9 illustrates an example method of generating a serializedrepresentation of a path in the loop-free routing topology, according toan example embodiment.

FIG. 10 illustrates the set of serialized representations describing theloop-free routing topology, according to an example embodiment.

FIG. 11 illustrates implementing the loop-free routing topology based onpropagating the set of serialized representations from the destinationnetwork node to network nodes in the computing network, according to anexample embodiment.

FIG. 12 illustrates selective pruning of selected paths from the set ofserialized representations propagated from the destination network nodeby network nodes within the loop-free routing topology, according to anexample embodiment.

FIG. 13 illustrates an example method of the establishment of loop-freelabel switched paths by the network nodes receiving the set ofserialized representations from the destination network node, accordingto an example embodiment.

FIG. 14 illustrates an example serialized representation of abuttressing arc, according to an example embodiment.

FIG. 15 illustrates an example label switched forwarding table built inone of the network nodes of FIG. 6I for routing at a primary hierarchallevel, according to an example embodiment.

FIGS. 16A, 16B, and 16C illustrate example establishment ofnon-congruent paths in a loop-free routing topology having routing arcs,for bicasting of data packets toward a root node and/or multicastingdata streams from the root node, according to an example embodiment.

FIG. 17 illustrates an example method of creating the non-congruentpaths in the loop-free routing topology of FIGS. 16A-16C, according toan example embodiment.

FIG. 18 illustrates an example registration by network nodes, andmulticasting by a multicast source, via the non-congruent paths of FIGS.16A-16C, according to an example embodiment.

FIG. 19 illustrates example non-congruent paths used by network nodes inthe loopfree routing topology of FIGS. 16A-16C for bicasting datapackets, and for use in multicasting by the multicast source, accordingto an example embodiment.

FIGS. 20A and 20B illustrate additional routing arcs having networknodes executing collision avoidance for establishing non-congruentmulticast paths for multicast streams, according to an exampleembodiment.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

In one embodiment, a method comprises creating, in a computing network,a loop-free routing topology comprising a plurality of routing arcs forreaching a destination network node, each routing arc comprising a firstnetwork node as a first end of the routing arc, a second network node asa second end of the routing arc, and at least a third network nodeconfigured for routing any network traffic along the routing arc towardthe destination node via any one of the first or second ends of therouting arc, the loop-free routing topology providing first and secondnon-congruent paths; and forwarding bicasting data, comprising a datapacket in a first direction from a network node and a bicasted copy ofthe data packet in a second direction from the network node,concurrently to the destination node respectively via the first andsecond non-congruent paths.

In another embodiment, an apparatus comprises a processor circuit and anetwork interface circuit. The processor circuit is configured forcreating, in a computing network, a loop-free routing topologycomprising a plurality of routing arcs for reaching a destinationnetwork node, each routing arc comprising a first network node as afirst end of the routing arc, a second network node as a second end ofthe routing arc, and at least a third network node configured forrouting any network traffic along the routing arc toward the destinationnode via any one of the first or second ends of the routing arc, theloop-free routing topology providing first and second non-congruentpaths. The network interface circuit is configured for causingforwarding of bicasting data, comprising a data packet in a firstdirection from a network node and a bicasted copy of the data packet ina second direction from the network node, concurrently to thedestination node respectively via the first and second non-congruentpaths.

DETAILED DESCRIPTION

Particular embodiments enable efficient network traffic management froma source network node to a destination network node in a loop-freerouting topology having distinct paths, such as routing arcs, thatenable the network traffic to be routing along any of two ends of anypath. The particular embodiments apply the use of routing arcs to createat least two non-congruent paths within the loop-free routing topology:the term “non-congruent” is defined as any network path (between asource node and a destination node) that does not share any part of itspath (including any intermediate network nodes or connecting data links)with any other network path. Hence, two network paths are non-congruentif they do not share any data link or any intermediate network nodebetween the source node and the destination node. The use ofnon-congruent paths guarantees that any traffic that is bicastedconcurrently along the non-congruent paths from the source node willreach the destination node, even if there is a failure in one of thenon-congruent paths.

The non-congruent paths also can be used for multicasting, where amulticast registration bicasted by a multicast consumer is marked (i.e.,recorded) by each intermediate network node and forwarded along thenon-congruent paths. The destination network node (i.e., the multicastregistration destination) can utilize the non-congruent paths fortransmission of multicast streams via the non-congruent paths. Collisionmediation also is employed to ensure multiple multicast subscribers donot create overlapping multicast streams.

The following description will begin with describing creation of aplurality of routing arcs (omega arcs), followed by a description ofestablishment of omega label switched paths for reaching a destinationnetwork node. The description of the omega arcs and the omega labelswitched paths are described with respect to FIGS. 1 through 15. Theomega label switched paths enable fast rerouting to be implementedwithin the loop-free routing topology. The description of establishingthe non-congruent paths will be described with respect to FIGS. 16through 19.

Generating the Loop-Free Topology Using Routing Arcs

FIGS. 1 through 6I describe the generation of routing arcs in loop-freetopology using routing arcs, according to an example embodiment. Routingarcs can be generated in a loop-free routing topology that can guaranteethat any network node having at least two data links can reach thedestination network node via at least one of the routing arcs: thenetwork traffic can be instantaneously rerouted toward another end of arouting arc in response to detected data link failure (or network nodefailure), based on reversing an identified reversible link within therouting arc. The reversal of the identified reversible link within therouting arc also is guaranteed to not introduce any loops into theloop-free routing topology.

In one embodiment, the loop-free routing topology is represented bygenerating a set of serialized representations describing the loop-freerouting topology. In particular, each serialized representationdescribes one of the paths (e.g., routing arcs), in the loop-freerouting topology; for example a serialized representation of a givenpath or routing arc can resemble a source route path from one end of thepath (e.g., a first junction node of a routing arc) to another end ofthe path (e.g., a second junction node of a routing arc). Hence, the setof the serialized representations provides a complete representation ofall the paths in the loop-free routing topology.

Moreover, any network node receiving the set of serializedrepresentations can establish multiple paths for reaching thedestination network node, for example in the form of loop-free labelswitched paths for reaching the destination network node. Hence, thepropagation of the serialized representations throughout the networknodes in the computing network enables the automatic deployment of labelswitched paths by the network nodes.

In particular, the following description with respect to FIGS. 1 through6I describe an apparatus creating, in a computing network, a loop-freerouting topology comprising a plurality of routing arcs for reaching adestination network node: each routing arc comprises a first networknode as a first end of the routing arc, a second network node as asecond end of the routing arc, and at least a third network nodeconfigured for routing any network traffic along the routing arc towardthe destination network node via any one of the first or second ends ofthe routing arc.

A junction node is defined as a network node (e.g., a computing networkhaving a prescribed network topology, for example a prescribed ringtopology) identified based on its relative position within a networktopology) that has at least two data links providing respectivenon-congruent paths for reaching the destination network node: the term“non-congruent” in this specification and the attached claims requiresthat the paths from a junction node do not share any common data linkfor reaching the destination network node, rather each link belonging toone path (from the junction node to the destination network node) isdistinct and independent of any link belonging to the secondnon-congruent path (from the junction node to the destination networknode).

A data link of a junction node also can be deemed a “reversible link” ifthe data link enables an endpoint of the data link (i.e., a network nodethat is directly coupled to the junction node via the data link) tobecome a junction node having its own non-congruent paths for reachingthe destination network node.

In one embodiment, one or more junction nodes coupled by a reversiblelink can be logically connected in sequence to create a routing arc as afirst infrastructure arc having its two corresponding ends terminatingat the destination network node. Additional routing arcs can be createdthat have at least one end terminating in a previously-createdinfrastructure arc (e.g., the first infrastructure arc), and the otherend terminating in either a previously-created infrastructure arc or thedestination network node, where each new routing arc includes networknodes that are distinct from any of the network nodes inpreviously-generated routing arcs. In other words, a network nodeassigned to one existing routing arc cannot later be assigned to anotherrouting arc (except if the network node is a junction between theexisting routing arc and an end of the newer routing arc).

In another embodiment, a routing arc can be created as an infrastructurearc based on identifying, within a first directed acyclic graph forreaching the destination network node, one or more junction nodes havingreversible links for reaching the destination network node vianon-congruent paths; a second routing arc can be created based onidentifying, within the first directed acyclic graph, a second directedacyclic graph for reaching one of the junction nodes (as one end of thesecond routing arc) of the first directed acyclic graph, and identifyingwithin the second directed acyclic graph a new junction node having anew non-congruent path for reaching the destination network node that isdistinct from the one junction node, the new junction node providing thenew non-congruent path via the second end of the second routing arc thatterminates either at the destination network node or apreviously-created infrastructure routing arc.

Hence, example embodiments enable any network node having two data linksin the loop-free routing topology to be guaranteed reachability to thedestination network node, even if any one data link in the loop-freerouting topology encounters a failure, based on reversing an identifiedreversible link.

FIG. 1 illustrates an example loop-free routing topology 10 comprising aplurality of routing arcs 12 for reaching a destination network node 14,according to an example embodiment. Each routing arc 12 comprises aplurality of network nodes 16 each having at least two data links forreaching adjacent network nodes. As apparent from FIG. 1, the loop-freerouting topology 10 guarantees that any network node 16 along any pointof any arc 12 (illustrated by “X” in FIG. 1) has at least twonon-congruent paths for reaching the destination network node 14,guaranteeing reachability to the destination network node 14 even if alink failure is encountered in the loop-free routing topology 10. Theterm “node” in the specification and claims can refer to a networkdevice or a network having a prescribed topology, for example aring-based network having a prescribed ring topology.

As described in further detail below with respect to FIGS. 6A-6I, eachrouting arc (e.g., “ARC1” of FIG. 6I) 12 comprises at least threenetwork nodes 16, namely a first network node (e.g., network node “K” ofFIG. 6I) 16 designated as a first end of the routing arc, a secondnetwork node (e.g., network node “J” of FIG. 6I) 16 designated as asecond end of the routing arc, and at least a third network node (e.g.,network node “M” of FIG. 6I) 16 identified as a junction node andconfigured for routing any network traffic along the routing arc towardthe destination network node 14 via any one of two availablenon-congruent paths provided by the first or second ends of the routingarc. Hence, network traffic can be forwarded along at least one of therouting arcs 12 to the destination network node 14.

As illustrated in FIG. 6I, the first and second ends of each routing arc12 each terminate at a “safe network node”, for example either thedestination network node 14, another network node directly coupled tothe destination network node (e.g., network node “A” or network node“B”), or a junction node of another routing arc. A network node (e.g.,“A” of FIG. 6I) directly coupled to the destination network node “R” 14is referred to as an “heir network node”. Hence, a “safe network node”can be any one of the destination network node 14, an heir network node(e.g., “A” or “B” of FIG. 6I), or a junction node having twonon-congruent paths for reaching the destination network node. Forexample, both ends of the routing arc “ARC2” 12 terminate at thedestination network node “R” 14, also referred to as the “root networknode” or “root node”; a first end of the routing arc “ARC3” 12terminates at the heir network node “A” 16, and a second end of therouting arc “ARC3” 12 terminates at the junction node “C” of the routingarc “ARC2” 12. The routing arc “ARC2” 12 also can be referred to as the“root arc”, since both ends of the routing arc “ARC2” 12 terminate atthe destination network node “R” 14

In an example embodiment illustrated in FIG. 5 and FIG. 6I, each routingarc comprises one and only one arc cursor (18 of FIG. 6I) that providesexclusive control of directing the network traffic along the routingarc. One and only one junction node of the routing arc (i.e., one andonly one network node assigned a position within the routing arc as ajunction node) has possession of the arc cursor 18 at any given time:the junction node having possession of the arc cursor 18 can control thenetwork traffic along the corresponding routing arc 12 based onpossession of the arc cursor 18. In particular, the junction node (e.g.,“J” of FIG. 6I) having possession of the arc cursor 18 can directnetwork traffic away from itself along either of its outwardly-orientedlinks toward the ends of the routing arc (e.g., “ARC2”) 12. Hence, ajunction node having possession of the arc cursor 18 (also referred toas an “arc cursor node”) has exclusive control over routing the networktraffic along the corresponding routing arc, based on the junction noderouting the network traffic away from itself along one of itsoutwardly-oriented links.

A second junction node (i.e., another network node a position within therouting arc as a junction node) can gain possession of the arc cursor(e.g., from the first junction node of the routing arc) based on adetected failure in the corresponding routing arc, for continued routingof the network traffic in the corresponding routing arc despite thedetected failure. For example, the junction node “F” of the routing arc“ARC2” 12 can gain possession of the corresponding arc cursor 18 thatwas previously owned by the junction node “J”, based on a detectedfailure in the link “F-C” between network nodes “F” and network node“C”, enabling the network node “F” to reverse the link “F-J” toward thenode “J” for continued reachability toward the destination network node“R” 14 (see FIGS. 6H and 6I). Hence, the second junction node (e.g., “F”of FIGS. 6H and 6I), upon gaining possession of the arc cursor 18 thatprovides exclusive control of directing the network traffic along therouting arc (e.g., “ARC2”) 12, can reverse one of the connectedreversible links without creating a loop in the loop-free routingtopology 10. Hence, data traffic along a routing arc (e.g., “ARC2”) 12can be instantaneously rerouted for continued routing in the routing arc12 toward the destination network node 14 in response to a detectedfailure in the routing arc (e.g., failure in link “F-C”), based on ajunction node (e.g., “F”) in the routing arc gaining possession of therouting arc 18 previously owned by another junction node (e.g., “J”) inthe same routing arc 12.

FIGS. 2 and 6A illustrate an example apparatus 20 for creating theloop-free routing topology 10 of FIGS. 1, 6I, and 16-25, according to anexample embodiment. The apparatus (i.e., device, machine) can beimplemented as a router, a centralized server, a network managemententity, etc. that executes the disclosed operations for creating theloop-free routing topology 10, and distributing relevant routing arcparameters to each of the network nodes implementing the loop-freerouting topology 10 as network nodes 16 within the topology 10. Theapparatus 20 is a physical machine (i.e., a hardware device) configuredfor implementing network communications with other physical machines 14,16 via data links establishing a link layer mesh topology network 44(see FIG. 2).

As illustrated in FIG. 2, the apparatus 20 includes a processor circuit22, a device interface circuit 24, and a memory circuit 26. Theprocessor circuit 22 is configured for creating, for a computingnetwork, the loop-free routing topology 10 comprising the routing arcs12 for reaching the destination network node 14. The memory circuit 26is configured for storing parameters associated with the routing arcs 12in a state table 54 and/or a topology table 56, described in furtherdetail below with respect to FIGS. 4 and 5. The device interface circuit24 is configured for outputting at least selected parameters associatedwith the routing arcs 12 to a second apparatus, for deployment of theloop-free routing topology 10: the second apparatus can be a networkmanagement entity for configuring the network nodes 16, or at least oneof the network nodes 16 that can be configured directly by the apparatus20. Hence, the output by the device interface circuit 24 of the selectedparameters for deployment of the loop-free routing topology 10 causingthe network traffic in the computing network to be forwarded along atleast one of the routing arcs to the destination network node.

Any of the disclosed circuits of the apparatus 20 (including theprocessor circuit 22, the device interface circuit 24, the memorycircuit 26, and their associated components) can be implemented inmultiple forms. Example implementations of the disclosed circuitsinclude hardware logic that is implemented in a logic array such as aprogrammable logic array (PLA), a field programmable gate array (FPGA),or by mask programming of integrated circuits such as anapplication-specific integrated circuit (ASIC). Any of these circuitsalso can be implemented using a software-based executable resource thatis executed by a corresponding internal processor circuit such as amicroprocessor circuit (not shown) and implemented using one or moreintegrated circuits, where execution of executable code stored in aninternal memory circuit (e.g., within the memory circuit 26) causes theintegrated circuit(s) implementing the processor circuit 22 to storeapplication state variables in processor memory, creating an executableapplication resource (e.g., an application instance) that performs theoperations of the circuit as described herein. Hence, use of the term“circuit” in this specification refers to both a hardware-based circuitimplemented using one or more integrated circuits and that includeslogic for performing the described operations, or a software-basedcircuit that includes a processor circuit (implemented using one or moreintegrated circuits), the processor circuit including a reserved portionof processor memory for storage of application state data andapplication variables that are modified by execution of the executablecode by a processor circuit. The memory circuit 26 can be implemented,for example, using a non-volatile memory such as a programmable readonly memory (PROM) or an EPROM, and/or a volatile memory such as a DRAM,etc.

Further, any reference to “outputting a message” or “outputting apacket” (or the like) can be implemented based on creating themessage/packet in the form of a data structure and storing that datastructure in a tangible memory medium in the disclosed apparatus (e.g.,in a transmit buffer). Any reference to “outputting a message” or“outputting a packet” (or the like) also can include electricallytransmitting (e.g., via wired electric current or wireless electricfield, as appropriate) the message/packet stored in the tangible memorymedium to another network node via a communications medium (e.g., awired or wireless link, as appropriate) (optical transmission also canbe used, as appropriate). Similarly, any reference to “receiving amessage” or “receiving a packet” (or the like) can be implemented basedon the disclosed apparatus detecting the electrical (or optical)transmission of the message/packet on the communications medium, andstoring the detected transmission as a data structure in a tangiblememory medium in the disclosed apparatus (e.g., in a receive buffer).Also note that the memory circuit 23 can be implemented dynamically bythe processor circuit 22, for example based on memory address assignmentand partitioning executed by the processor circuit 22.

The following definitions are provided prior to a description of themethods for creating the routing arcs.

A routing arc is defined as a double ended reversible path. A reversiblearc is defined as a routing arc containing one or more reversible links,and the reversible arc can contain one or more non-reversible links ateach end of the arc. Data links that are labeled “Rev” in FIGS. 6Bthrough 6H are reversible links, and links that are not labeled “Rev” inFIGS. 6B through 6H are not reversible; hence, the end of a routing arccan be identified based on one or more successive non-reversible links,with one or more reversible links internal to the non-reversible links.A collapsed arc (e.g., “ARC1”, “ARC3”, “ARC6”, “ARC7”, and “ARC8” ofFIG. 6I) is defined as a routing arc 12 having no reversible link andconsisting of a one network node that has fixed (i.e., non-transferable)possession of the arc cursor 18, and two other network nodes 16 nodesserving as respective ends of the collapsed arc. For example, thecollapsed arc “ARC1” 12 is illustrated in FIG. 5 and FIG. 6I asconsisting of the network nodes “J”, “M”, and “K”, where the networknode “M” has fixed possession of the arc cursor 18 between the networknodes “J” and “K” at the respective ends of the collapsed arc “ARC1” 12.

A link designated with an arrow at one end and having a designation of“SPF” represents a link as found in a tree generated according to aconventional routing protocol such as Open Shortest Path First (OSPF),such that the network node at the end of the arrow represents a shortestpath first (SPF) successor of the network node at the tail end of thearrow (e.g., network node “A” in FIG. 6A is the SPF successor to networknodes “C” and “D”). Any link at the edge of the arc (i.e., thatterminates the arc and connects the arc either to a second arc or to thedestination) will be designated using the arrow at one end. A linkdesignated with “TOP” (representing “then other path”) represents a linkthat has not been chosen by OSPF because it is not the shortest pathsuccessor, but that can be used as an alternate next hop (i.e., afeasible successor), for example for generating a directed acyclic graph(DAG) (see, e.g., U.S. Pat. No. 7,656,857).

As described previously, the network node having possession of the arccursor can decide in which direction along the arc network trafficshould be forwarded. Hence, a network node is determined to be a “safenetwork node” if the “safe network node” can forward network traffic ineither direction along the arc (i.e., the “safe network node” can safelyforward any packet in one direction along the arc even if the otherdirection fails).

A link designated with the reference “?-S” represents a candidate linkthat is unresolved for a junction node “S” that is identified as thenearest safe network node for the network node via the link having thedesignation “?-S”: reference to a candidate link denotes a transientstate when the two ends of a routing arc have not yet been established,and it is not yet established whether the candidate link is to be usedin the formation of a routing arc. As described in further detail belowwith respect to FIGS. 6B-6F, the links designated with the reference“?-S” also identify a subDAG (i.e., a DAG within a DAG) for reaching thesafe node “S”.

A link designated with the reference “Rev” indicates a reversible linkconnecting two network nodes that are within a routing arc 12: asillustrated in FIGS. 6H and 6I, a network node (e.g., “J”) having atleast one reversible link is within the middle of the arc, and can havepossession of the arc cursor 18 for the corresponding routing arc. Asillustrated in FIGS. 6H and 6I, data links at the edge of a routing arc(e.g., that terminates at a first arc and enters into a second arc, orthat terminates at the destination node D) are directed away from themiddle of the routing arc (and the junction node having possession ofthe corresponding arc cursor 18) 12, and the data links at the edge of arouting arc 12 are not reversible.

A link designated with a square-shaped or diamond-shaped block at oneend (e.g., “M□→J” in FIG. 6C) indicates a blocked link that is notreversible, where the destination network node (e.g., network node “J”in FIG. 6C) cannot send any data traffic to the other sourcing networknode (e.g., “M” of FIG. 6C), but the sourcing network node (e.g., “M” ofFIG. 6C) can send data traffic to the destination network node (“J”) viathe link (“M□→J”). Blocked links are used during computation to preventany formation of loops.

As described in further detail below, data links are oriented away fromthe junction node having possession of the arc cursor toward the edgesof the routing arc 12, and link orientation of a reversible link can bechanged by moving the arc cursor 18 (i.e., passing ownership of thecursor from one network node to another network node).

Routing arcs 12 are built between network nodes identified as junctionnodes. A junction node 68 is a network node connected to two or moresafe network nodes (described below) over non-congruent paths (i.e., nosingle point of failure can stop reachability from the junction node tothe root node). An edge junction is defined as a junction node 68terminating one and only one reversible link, where the edge junctioncan have multiple nonreversible links oriented both inwards and/oroutwards. An intermediate junction is defined as a junction node 68 thatterminates two and only two reversible links, where all other linkscoupled to the intermediate junction are oriented inwards to avoidloops: a link can be safely reversed towards an intermediate junction.Hence, an intermediate junction consists of two reversible links andzero or more inward oriented links from any other network node. Acollapsed arc does not have any intermediate junction, and an edgejunction can belong to one or more collapsed arcs.

A root network node 14 is defined as a single network node in a network(i.e., a “destination network node”) that must be accessed to reach aresource, i.e., there never can be a second path that can bypass theroot network node to reach the resource. Calculation of routing arcs 12begins with identification of a root node (i.e., the destination node)14 for a given routing topology 10. Examples of a root node 14 caninclude a head end of an autonomous directed acyclic graph within therouting arcs 12, a gateway to another network, or any identifiabledestination. All of the root links always are oriented inwards towardthe root node 14 and resolved.

An “heir” network node is a network node that is directly connected to aroot network node 14. As illustrated in FIGS. 1 and 6A-6I, a destinationnetwork node serving as a root network node 14 must have at least twoheir network nodes (e.g. network nodes “A” and “B” of FIGS. 6A-6I) thatare directly connected to a root network node 14: if a root network nodehas only one heir node, then the heir network node is designated as thenew root node (based on the definition of a root network node as havingno second path to reach a resource). The heir network node is used toidentify a safe network node: if a network node can reach the root nodealternatively via either a first heir network node or a second heirnetwork node, then the network node is considered a safe network nodebecause it can route a packet to the root via two non-congruent paths,namely either via the first heir network node or via the second heirnetwork node, guaranteeing that reachability if one of the links towardthe root node is broken.

A leaf network node is a node that has one and only one data link: aleaf node cannot be a junction node and cannot belong to a routing arc12. The data link coupled to the leaf network node is always orientedoutwards (i.e., away from the leaf network node) and resolved.

A safe network node is a designation used to identify any one of a rootnetwork node 14, an heir node (e.g., “A” or “B” of FIGS. 6A-6I), or ajunction node. Hence, a junction node is a network node that is neithera root network node 14 nor an heir network node, but that is a safenetwork node because it has two or more non-congruent paths to the rootnetwork node such that no single point of failure can cut off thejunction node from the root network node. A network node can beidentified as a junction node if the direction of a reversible link mustbe turned to obtain the alternate path.

Hence, a network node connected to an heir network node and the rootnetwork node is a junction node; a network node connected to twodifferent heir network nodes is a junction node; a network nodeconnected to an heir network node and a junction node also is a junctionnode; a network node connected to two different junction nodes also is ajunction node. Since the root network node, the heir network node, andthe junction node each are defined as safe network nodes, then a networknode connected to two different safe network nodes is a junction node; anetwork node that has non-congruent paths to at least two different safenetwork nodes is a junction node (the junction node can be considered to“see” to safe network nodes, and then hide them so long as they are onlyreachable from via that junction node); a network node that can only seeone junction node is within the “subDAG” that junction node and can betracked as such.

Hence, a data packet must follow along a routing arc 12, and can exit arouting arc 12 only via an edge junction at one of the ends of therouting arc 12. Consequently, a data packet can reach the root node(i.e., the destination node) 14 based on traveling along one or morerouting arcs 12.

FIG. 3A is a diagram illustrating an example method by the apparatus 20of FIG. 2 that includes creating a loop-free routing topology 10comprising routing arcs 12 for reaching a destination network node 14,according to an example embodiment. FIGS. 3B, 3C and 3D illustrate analternate method by the apparatus 20 for creating the loop-free routingtopology 10, according to another example embodiment. The operationsdescribed with respect to any of the Figures (e.g., FIGS. 3A-3D, FIGS.8-11, 13, and 17-18, described below) can be implemented as executablecode stored on a computer or machine readable non-transitory tangiblestorage medium (e.g., floppy disk, hard disk, ROM, EPROM, EEPROM,nonvolatile RAM, CD-ROM, etc.) that are completed based on execution ofthe code by a processor circuit implemented using one or more integratedcircuits; the operations described herein also can be implemented asexecutable logic that is encoded in one or more non-transitory tangiblemedia for execution (e.g., programmable logic arrays or devices, fieldprogrammable gate arrays, programmable array logic, application specificintegrated circuits, etc.) by a machine or device.

Referring to FIG. 3A, the processor circuit 22 can create the loop-freerouting topology 10 based on creating in operation 30 the first routingarc 12 as an infrastructure arc (i.e., a routing arc that is not acollapsed arc): the first routing arc 12 (e.g., 12 a or 12 b of FIG. 1)created in operation 30 must have both ends terminating at thedestination network node (i.e., the root node) 14. The processor circuit22 can create in operation 32 the next routing arc 12 (after the firstrouting arc 12 a or 12 b created in operation 30) as an infrastructurearc (e.g., 12 c after 12 b) or a collapsed arc (e.g., 12 d after 12 a),subject to the following rules: (1) the next routing arc 12 underconstruction must terminate either in the junction node of an existingrouting arc or at the destination network node 14 (e.g., routing arc 12c terminates one end at the routing arc 12 b and terminates the otherend at the destination network node 14); and (2) except for an end of arouting arc terminating in the junction node of an existing routing arc,the routing arc under creation must be made from network nodes that arenot already in an existing routing arc; in other words, in rule (2) thenext routing arc 12 includes network nodes that are distinct from any ofthe network nodes of the existing routing arcs. The next routing arc 12can be constructed by the processor circuit 22 in operation 34, untilall the routing arcs have been completed for all network nodes having atleast two data links. The processor circuit 22 identifies in operation36 any leaf network nodes consisting of only one data link, andimplements in operation 38 the loop-free routing topology 10 constructedin operations 30, 32, 34, and 36.

As illustrated operations 30, 32, and 34, the first routing arc 12(e.g., 12 a or 12 b) preferably is created prior to any other routingarc 12 (e.g., 12 c or 12 d); however, the routing arcs 12 in FIG. 1 canbe created in any arbitrary order, so long as the rules of operation 32are followed to guarantee no loop formation, where the “existing routingarc” refers to a set of network nodes that already are allocated foranother routing arc. Hence, alternative techniques for creating theloop-free routing topology 10 can be utilized, so long as: the routingarcs 12 do not intersect across each other during formation; or anyrouting arcs 12 that intersect across each other share the same cursorat the intersection point (e.g., a collapsed arc), causing all networktraffic to be directed away from the cursor point.

The processor circuit 22 can deploy in operation 38 the loop-freerouting topology 10 based on causing the device interface circuit 24 tooutput at least selected parameters associated with the routing arcs 12to at least one other apparatus (e.g., a network router, a networkmanagement apparatus, one or more network nodes, etc.), causing thenetwork traffic in the routing topology 10 to be forwarded along atleast one of the routing arcs 12 to the destination network node 14. Ifin operation 40 a link failure is detected (or a network node failure)in one of the routing arcs, for example by either the apparatus 20 or byone of the network nodes 14 or 16, the possession of the arc cursor 18can be gained (e.g., by transfer, reassignment, etc. by the processorcircuit 22) by the junction node adjacent to the link failure inoperation 42, enabling the junction node to control the network trafficbased on reversing one of the reversible links for continued routingtoward the destination network node 14 without creating a loop in theloop-free routing topology 10.

FIGS. 3B, 3C and 3D illustrate an example method by the processorcircuit 22 for creating the loop-free routing topology 10 comprising therouting arcs 12 of FIG. 6I, according to an example embodiment.

According to an example embodiment, a loop-free routing topology 10 canbe created in which an attempt is made to establish every network node,except the root network node and the heir network nodes, as junctionnodes, in order to guarantee that every network node has a shortest pathand an alternate path to a destination network node (i.e., the rootnetwork node) 14. This guarantee is established by creating routing arcs12 that terminate at safe network nodes. Since conventional techniquesfor generating a directed acyclic graph (DAG) does not guarantee thatevery node within a directed acyclic graph can be a junction, theexample embodiments enable a link to be reversed in response to adetected failure in a network node or network link, enabling immediatererouting of data traffic without route recalculation and without theformation of any loop in the topology. Hence, the example embodiment canestablish a loop-free routing topology of routing arcs for reaching aroot network node, the loop-free routing topology consisting of a rootnetwork node, two or more heir network nodes coupled to the root networknode, junction nodes, and zero or more leaf network nodes.

As described in further detail with respect to FIGS. 3B, 3C, 4-5 and6A-6I, the loop-free routing topology 10 is created based on: generatinga first directed acyclic graph for reaching the destination network node14; identifying (within the first directed acyclic graph) junction nodesthat have non-congruent paths for reaching the destination network node14; identifying reversible links between the junction nodes, along thenon-congruent paths, and that do not introduce or create a loop in theloop-free routing topology 10; and identifying at least one seconddirected acyclic graph within the first directed acyclic graph (alsoreferred to as a “subDAG”) for reaching one of the junction nodes, thesecond directed acyclic graph including an identified new junction nodehaving a new non-congruent path for reaching the destination networknode.

In this disclosure, links can be identified by the connecting networknodes, such that the link “A-R” refers to a wired or wireless link thatconnects the network node “A” to the next hop network node “R”: an arrowin a link designation can refer to an assigned direction (e.g., “A→R”and “R←A” equivalently identify a link directing traffic from node A tonode R), whereas the absence of an arrow in a link designation (e.g.,“A-R”) indicates that a direction has not yet been assigned to the link.

Referring to FIGS. 2 and 3B, the processor circuit 22 identifies thedata link layer topology 44 (FIG. 2) composed of wired or wireless datalinks (e.g., wired or wireless IEEE 802.11, Bluetooth, etc.) 46, andcreates in operation 50 a directed acyclic graph (DAG) 52 for reachingthe destination network node 14, illustrated in FIG. 6A. In particular,the link layer topology 44 of FIG. 2 includes network nodes 14 havingone or more link layer connections (e.g., wired or wireless IEEE 802links, Bluetooth links, etc.) interconnecting the network nodes, forexample in the form of a link layer (i.e., OSI Layer 2) mesh ofinterconnected network nodes. The directed acyclic graph 52 is generatedin operation 50 by the processor circuit 22, for example according tothe techniques described in U.S. Pat. No. 7,656,857, where the linkslabeled “SPF” identify the Shortest Path First (SPF) links in the SPFtopology, and the links labeled “TOP” identify the “then other path”(TOP) links overlying the SPF topology that result in the formation ofthe directed acyclic graph (DAG) toward the root node “R”. In summary,the directed acyclic graph is formed by the processor circuit 22 inoperation 50 based on identifying next hop nodes adjacent to the rootnode 14, and orienting the link of each next hop node toward the root.Secondary adjacent nodes (adjacent to each of the next hop nodes) arethen identified, and the paths from the next hop nodes to the associatedsecondary adjacent nodes are extended while orienting each of the linksbetween the secondary adjacent nodes and the next hop nodes toward thenext hop nodes. These operations are repeated recursively until thepaths extend to the furthest nodes in the network, and orienting thelinks back toward the root, for formation of the directed acyclic graph.

The SPF status of the SPF links are retained in the memory circuit 26during calculation of the routing arcs 12 with respect to FIGS. 6A-6I.FIG. 4 (consisting of FIGS. 4A, 4B, 4C and 4D) illustrate an examplestate table 54 that can be stored in the memory circuit 26 andconfigured for storing state changes in the network nodes and the datalinks during execution of the method in FIGS. 3B, 3C and 3D.

FIG. 5 illustrates an example topology table 56 that can be stored inthe memory circuit 26 and configured for storing arc topology attributesas the routing arcs are constructed for reaching the destination networknode (i.e., root network node “R”) 14. The processor circuit 22 canupdate the topology table 56 of FIG. 5 as the state table 54 is updated(e.g., upon identification of SPF links, identification of reversiblelinks, identification of junction nodes 68, upon creating a routing arc,etc.), where the ownership of an arc cursor 18 is identified by thejunction node 68 having the two outwardly oriented reversible links.Hence, the processor circuit 20 stores in operation 50 the SPF links 58in the topology table 56, and the non-SPF links (not shown in FIG. 5).

As illustrated in FIG. 6A, none of the network nodes in the DAG topology52 (except the network node “J”) has at least two non-congruent pathsfor reaching the root node “R”. Hence, the method of FIGS. 3B, 3C and 3Dprovides the creation of the routing arcs 12 in a loop-free routingtopology 10, the routing arcs enabling network traffic to be routedalong the routing arcs toward the destination root network node via anyone of the ends of the routing arcs.

After formation of the directed acyclic graph in operation 50, thegeneration of the routing arcs begins in operation 60 of FIG. 3B, wherethe SPF links are retained, while selectively modifying non-SPF links inthe directed acyclic graph as either unidirectional links or reversiblelinks. The status of each network node and each data link can be storedin a memory circuit.

The generation of the routing arcs in operation 60 begins withidentification of the root network node R and orienting unoriented linkstoward the root (A→R, B→R) as illustrated by the state change in thestate table of FIG. 4A at event 200, and initiating identification ofthe heir network nodes (e.g., node A) in operation 62. As described infurther detail below, the SPF nodes are successively analyzed by theprocessor circuit 22 in operation 62 for identification of subordinatedirected acyclic graphs (subDAGs) 64 (e.g., 64 a and 64 b of FIG. 6B)within the DAG 52 toward the destination network node 14. Each heirnetwork node link (e.g., A-R) that is coupled to the root network node14 is oriented outward toward the root network node and marked in thestate table 54 as resolved (A→R=Resolved) (event 202 of FIG. 4A). Eachneighbor network node is successively resolved in operation 66 toidentify any junction node within a subDAG 64 that has an alternatenon-congruent path for reaching the destination network node 14 via apath that is distinct from the subDAG 64; in other words, each neighbornetwork node 16 is successively resolved to identify, within a subDAG64, any junction nodes having at least two non-congruent paths forreaching the destination network node.

FIGS. 3C and 3D illustrate in further detail the operations executed bythe processor circuit 22 in operation 66. The operations of FIGS. 3C and3D are illustrated in the example form of “C code” for a function callentitled “resolve_neighbor(N)”, annotated with outline annotations toidentify nested operations. The apparatus 20 is omitted in FIGS. 6Bthrough 6I and FIG. 7 to simplify the illustrations, although it will beapparent that the apparatus 20 will be consistently present duringcalculation of the routing arcs 12 and creation of the loop-free routingtopology 10 (and load balancing of the loop-free routing topology 10 asdescribed below with respect to FIGS. 7-9).

The first operation in operation 70 is executed by the processor circuit22 if the neighbor node “N” under analysis is a leaf network nodeconsisting of one and only one data link. In this example, the currentstate of execution is state 202 of FIG. 4A, where the heir network node“A” is under analysis; hence, operation 70 is skipped because the heirnetwork node A is not a leaf node. Operation 72 is executed by theprocessor circuit 22 if the neighbor node “N” under analysis is not asafe node. In this current state of analyzing the heir network node “A”,operation 72 is skipped because the heir network node A is a safenetwork node (because it is an heir network node).

As described previously, the method executed by the processor circuit 22attempts to identify adjacent network nodes that are safe network nodes.Hence, any of the links (N-i) in the safe network node under analysis(e.g., Node N=Node A) that have not yet been labeled (i.e., areunlabeled) (e.g., D→A, C→A), are set initially to an unresolved statustoward the nearest safe node (D→A=“?-S”; C→A=“?-S”) to indicate that ithas not yet been determined whether the links (e.g., D→A, C→A) couplethe network node (e.g., node A) to another safe network node thatenables the link to be reversible, or whether the links couple thenetwork node to an unsafe network node (i.e., a network node that is nota safe network node as previously defined).

Each of the links (N-i) of the safe network node under analysis (e.g.,node N=node A) are resolved in operation 74 in order according to SPFidentification, namely links providing shortest path first, followed byoutgoing links, followed by incoming links. If in operation 76 a datalink is already resolved, the execution by the processor circuit 22proceeds to the next link in operation 74: note that the link A→R isresolved and ends in the root network node 14, identifying the link A→Ras terminating a routing arc 12. Operations 78 through 86 are currentlyskipped by the processor circuit 22 because the safe network node underanalysis (e.g., node N=node A) has no other unresolved outgoing links.The processor circuit 22 returns to the next link in operation 74,namely the incoming links.

If in operation 88 the processor circuit 22 determines the unresolvedlink under analysis (which is not an SPF link or an outgoing link) hasno assigned direction, the link is assigned an incoming direction todirect traffic toward the safe network node under analysis (e.g., NodeN=Node A). If the incoming link (e.g., D→A based on the initial directedacyclic graph) is marked to an unresolved status (e.g., D→A=“?-S”), theincoming link is marked to an unresolved status with respect to the safenetwork node under analysis (i.e., the link D→A is reset from “?-S” to“?-N”). Hence, the link “D→A” is reset to the status “?-A” (Node N=NodeA: D→A=“?-A”); the process is repeated in operations 74 and 88 by theprocessor circuit 22 for the next link of node A, hence the link “C→A”is reset in operation 88 to the status “?-A” (C→A=“?-A”), indicatingthat it has not yet been determined whether the links “D→A” and “C→A”are reversible to reach another safe network node (the links areoriented toward the nearest safe network node). Hence, the unresolvedstatus indicates that a routing arc cannot be formed yet because theunresolved link has not been found to be reversible toward an alternatenon-congruent path to the root network node. All the unresolved incominglinks in the subDAG toward the safe network node “N” (Node N=Node A) arerecursively relabeled in operation 88, resulting in the identificationof subDAG(A) 64 a of FIG. 6B via the links labeled “?-A”.

After all the links for the safe network node under analysis (e.g., NodeN=Node A) have been analyzed, the process of operations 62 and 66 ofFIG. 3B are repeated by the processor circuit 22 for the next nodehaving the shortest path (in the SPF computation of operation 50) to theroot network node “R” that has not yet been analyzed (e.g., heir networknode “B”) (event 204 of FIG. 4A). The network node “B” is identified bythe processor circuit 22 as an heir network node in operation 62, andthe root link “B→R” is identified by the processor circuit 22 as an SPFlink and oriented toward the root network node, and marked in the statetable 54 and the topology table 56 as a resolved SPF link in operation62. As illustrated by the execution by the processor circuit 22 ofoperation 66 in FIGS. 3C and 3D, since network node “B” is identified asan heir network node (and therefore a safe network node) having anadditional incoming link “K-B”, operations 70 and 72 are skipped by theprocessor circuit 22, and the processor circuit 22 changes the status ofthe link “K→B” in operation 88 in the state table 54 from “?-S” to “?-B”(K→B=“?-B”). The remaining incoming links are recursively resolvedtoward the nearest safe node “B”, resulting in the subDAG(B) 64 b ofFIG. 6B.

Processing continues by the processor circuit 22 in operation 62 of FIG.3B to the next node identified by the SPF computation as closest to theroot network node “R” that has not yet been analyzed, namely the networknode “K” (event 206 of FIG. 4A). Operation 70 of FIG. 3C is skippedbecause the network node “K” is not a leaf node. Hence, the network node“K” is not a safe network node because it does not yet have twonon-congruent paths to the root network node “R”, rather the networknode “K” currently has only the SPF link “K→B” to the safe network node“B”. Hence, all the non-SPF links (e.g., M-K and J-K) are assigned bythe processor circuit 22 to be oriented incoming to the current networknode “K” under analysis, and the links are labeled by the processorcircuit 22 in operation 72 as unresolved to the nearest safe networknode (e.g., M→K=“?-B”; J→K=“?-B”).

Hence, the current set of analyzed network nodes include the networknodes “A”, “R”, “B”, and “K”.

The method continues by the processor circuit 22 in operation 62 of FIG.3B and 104 with the next SPF network node, identified as a network node“M” which is not a safe network node (event 208 of FIG. 4A). Operation70 skipped by the processor circuit 22, the node “M→K” is identified asthe SPF link, and in operation 72 the link “J-M” is assigned in thestate table 54 in operation 72 as an incoming link having an unresolvedstatus to the nearest safe network node “B” (J→M=“?-B”).

The next network node chosen by the processor circuit 22 in operation 62of FIG. 3B from the SPF nodes is the network node “D” (event 210), thelink “D→A” is identified as the SPF link, operation 70 is skipped by theprocessor circuit 22, and since the network node “D” is not a safenetwork node, the remaining non-SPF links are assigned by the processorcircuit 22 as incoming to the network node “D” in operation 72, andlabeled in the state table 54 as unresolved to the nearest safe networknode “A” (C→D=“?-A”; E→D=“?-A”; L→D=“?-A”). As described in furtherdetail below, the cost of each of the non-SPF links for each of thenetwork nodes can be tracked for later analysis. The method is repeatedby the processor circuit 22 for the network node “C” (event 212),resulting in the link “C→A” identified as the SPF link and the labelingof the links E→C=“?-A” and F→C=“?-A” in the state table 54 in operation72.

The next network node chosen in operation 62 of FIG. 3B from the SPFnodes is the network node “L” (event 214). Operation 70 is skipped bythe processor circuit 22, and since the network node “L” is not a safenetwork node, link L→D is identified by the processor circuit 22 as theSPF link, the link “E-L” is assigned as incoming to the network node“L”, and labeled in the state table 54 as unresolved to the nearest safenetwork node “A” (“E→L”=“?-A”) in operation 72.

The next network node chosen by the processor circuit 22 in operation 62of FIG. 3B from the SPF nodes is the network node “E” (event 216).Operation 70 is skipped, and since the network node “E” is not a safenetwork node, in operation 72 the link E→C is identified by theprocessor circuit 22 as an SPF link, and all the non-SPF links areoriented as incoming to the network node “E” and labeled as unresolvedto the nearest safe network node “A”, resulting in the labeling of thelinks F→E=“?-A”, G→E=“?-A”, and H→E=“?-A” in the state table 54.

The next network node by the processor circuit 22 in operation 62 ofFIG. 3B and 104 from the SPF nodes is the network node “F” (event 218).Operation 70 is skipped by the processor circuit 22, and since thenetwork node “F” is not a safe network node, in operation 72 the linkF→C is identified as an SPF link, and all the non-SPF links are orientedas incoming to the network node “F” and labeled by the processor circuit22 as unresolved to the nearest safe network node “A”, resulting in thelabeling of the links H→F=“?-A”, I→F=“?-A”, and J→F=“?-A” in the statetable 54.

The next network node chosen by the processor circuit 22 in operation 62of FIG. 3B from the SPF nodes is the network node “N” (event 220). Thenetwork node “N” is identified by the processor circuit 22 as leafnetwork node based on its one and only one link N→L; hence, the link“N-L” is marked in the state table 54 as resolved (N→L=Resolved) inoperation 70.

The next network node chosen by the processor circuit 22 in operation 62of FIG. 3B from the SPF nodes is the network node “G” (event 222).Operation 70 is skipped, and since the network node “G” is not a safenetwork node, in operation 72 the link G→E is identified by theprocessor circuit 22 as an SPF link, and the non-SPF link H-G isoriented as incoming to the network node “G” and labeled as unresolvedto the nearest safe network node “A”, resulting in the labeling of thelink H→G=“?-A” in the state table 54.

The next network node chosen in operation 62 by the processor circuit 22from the SPF nodes is the network node “H” (event 224). Since thenetwork node “H” is not a safe network node, in operation 72 the linkH→F is identified by the processor circuit 22 as an SPF link, and thenon-SPF link I-H is oriented as incoming to the network node “H” andlabeled as unresolved to the nearest safe network node “A”, resulting inthe labeling of the link I→H=“?-A” by the processor circuit 22 in thestate table 54.

The next network node chosen by the processor circuit 22 is the networknode “I” (event 226. Since the network node “I” is not a safe networknode, in operation 72 the link I→F is identified by the processorcircuit 22 as an SPF link in the state table 54. As describedpreviously, each of the SPF links 58 also can be labeled by theprocessor circuit 22 in the topology table 56 of FIG. 5.

As apparent from the foregoing description with respect to FIG. 6B, theidentification of SPF links and unresolved links enables identificationof the shortest path tree plus alternative unresolved links that can beused for identification of an alternate non-congruent path that isdistinct from the shortest path to the root network node. The followingdescription with respect to node “J” demonstrates how the identificationof the alternative unresolved links enables identification of one ormore junction nodes within the subDAGs 64 a and 64 b for formation ofthe arc routing topology.

In particular, the following description illustrates the identificationwithin the DAG 52 (two or more) junction nodes, and reversible links(labeled “Rev”) between the unction nodes and that can be reversed alongone of the non-congruent paths of the junction nodes, withoutintroducing a loop into the loop-free topology. In addition, theidentification of a junction node in one subDAG (64 a or 64 b) that hasa non-congruent path for reaching the destination network node 14(distinct from the junction node serving as the root of the subDAG)enables formation of another infrastructure arc overlying a firstinfrastructure arc.

As illustrated with respect to FIGS. 6B and 6C, the next network nodechosen by the processor circuit 22 in operation 62 of FIG. 3B from theSPF nodes is the network node “J” (event 228). The node “J” isidentified by the processor circuit 22 as a safe network node because itcan be classified as a junction node 68, illustrated in FIGS. 3C and 3D.The node “J” can be identified by the processor circuit 22 as a junctionnode because it has two non-congruent paths for reaching a safe node(e.g., the root network node “R”) via the link J→F (labeled asunresolved to the nearest safe network node “A”, i.e., “?-A”), and/orthe link J→K (labeled as the SPF link unresolved to the nearest safenetwork node “B”, i.e., “?-B”). Hence, the non-congruent paths providedby the links J→F and J→K are identified by the respective and distinctlabels “?-A” and “?-B” identifying reachability to the root network node“R” via the respective safe network nodes “A” and “B”.

Operation 72 is skipped by the processor circuit 22 because the networknode “J” is identified as a junction node. Each of the data links of thenetwork node “J” are resolved in operation 74 in a prescribed order,namely SPF link first, then any outgoing link(s), then any incominglink(s). Hence, the link J→K is the SPF link and therefore the firstlink to be analyzed in operation 74. Since the link J→K is unresolved inoperation 76, the outgoing link J→K in operation 78 does not end inanother junction, hence operation 80 is skipped.

Referring to FIG. 3D, the processor circuit 22 determines whether toexecute operation 82 if the network node is not yet an intermediatejunction. Operation 82 is executed by the processor circuit 22 for thelink J→K of the network node “J” because if the link J→K, if reversed,the link reversal would enable the endpoint network node “K” to become ajunction node, i.e., the endpoint network node “K” could now have analternate path to the root network node “R” via the safe network node“A” (e.g., if the label of link J→K was changed upon link reversal from“?-B” to “?-A”); hence, the link J→K enables the node “J” to become ajunction and does not cause any pruning of the SPF link J→K.Consequently, the link J→K is resolved by marking the link in the statetable 54 as reversible (“Rev”) by the processor circuit 22 in operation82. The labeling of the link J→K as reversible (“Rev”) is illustrated inFIG. 6D.

Since the node “J” is an edge junction toward the safe network node “B”,operation 84 is executed by the processor circuit 22 to prevent theformation of a loop via the outward link “J→M” in the event that thelink J→K is ever reversed to K→J in order to reach the safe network node“A”; hence, since “J→M”=“?-B” is an outward link satisfying thecondition “?-S” (where S=B), the outward link “J→M”=“?-B” is pruned inoperation 84 by blocking the path toward node “M” (note the square □ inFIG. 6C at the end of the link intersecting with network node “M”,indicating traffic from network node “J” to network node “M” isblocked); the direction of the pruned (i.e., blocked) link J-M isreversed and relabeled by the processor circuit 22 as unresolved inwardtoward the new safe network node (junction node), i.e., M□→J=“?-J” inthe state table 54.

Also note that if in operation 84 another link existed (e.g., J→B) (notshown in FIG. 6B) as an unresolved link toward the safe node “B”(J→B=“?-B”), that link (J→B) could be pruned at both ends (i.e.,removed: J□-□B) in order to avoid the possibility of any loops in theevent of any link reversal.

Hence, in order to avoid loops a junction node in a routing arc can haveone and only one link toward a first end of the arc, and one and onlyone other link toward a second end of the arc. The link J→F isunaffected in operation 84 because J is an edge junction toward safenode “B” (via J→K), whereas the node J→F is toward the other safenetwork node “A”.

Operation 86 is skipped by the processor circuit 22 for junction node“J” because it is not yet an intermediate junction because it does notyet have two reversible links. Operation 88 is skipped because the SPFlink J→K is assigned as an outgoing link.

Execution of operation 78 by the processor circuit 22 with respect tothe SPF link J→K (“Rev”) continues in operation 90, which recursivelycalls execution of operation 66 (as illustrated in FIGS. 3C and 3D) forthe endpoint of the link J→K, namely the network node “K”, in order toextend the arc along the shortest path; in other words, the operationsof FIGS. 3C and 3D are executed based on the change in status of thelink J→K to a reversible link (event 230 of FIG. 4B). Hence, operation70 is skipped for the J→K link endpoint node “K” (which is not a leafnode). The J→K link enables the endpoint node “K” to now become ajunction node since the link J→K link is now reversible, hence thenetwork node “K” as a junction node (specifically, an edge junction),and now a safe network node, hence operation 72 is skipped by theprocessor circuit 22. Each of the safe network node “K” links areresolved in operation 74, starting with the SPF link K→B: operation 76is skipped by the processor circuit 22 because the SPF link K→B=“?-B” isnot yet resolved. The SPF link K→B is an outgoing link, hence operation78 is executed by the processor circuit 22: operation 80 is not executedby the processor circuit 22 because the node K→B does not end in anotherjunction node (i.e., the heir network node “B” is not a junction node).Operation 82 is executed by the processor circuit 22 because networknode “K” is not an intermediate junction node yet, and the link K→Benables the end point network node B to become a junction node, hencethe node K→B is labeled as reversible, K→B=“Rev” in operation 82, toenable the heir network node “B” to become labeled by the processorcircuit 22 as a junction node in the state table 54.

In operation 84 the node N=K is now an edge junction toward node “B”,and there is no outward unresolved link to be pruned; however, the linkM→K is relabeled by the processor circuit 22 from “?-B” to “?-K” in thestate table 54. In operation 86 the network node “K” is now identifiedby the processor circuit 22 as an intermediate junction having tworeversible links J→K=“Rev” and K→B=“Rev”; however, there are no otheroutward links other than the two reversible links, hence no pruning ofother outward links is needed.

Hence, the resolution of the link J→K at node J as a reversible linkcauses resolution of the link endpoint K to be recursively resolved bythe processor circuit 22 at operation 90, resulting in the resolution ofreversible link K→B at node K. The resolution of the reversible link K→Bat node K causes in operation 90 the recursive resolution by theprocessor circuit 22 of the link endpoint B (event 232).

The heir network node B is identified as an edge junction based on thereversible link K→B, and since the SPF link B→R is to the root, the heirnetwork node “B” is identified as the end of a routing arc. Theresolution of node B causes the recursive execution by the processorcircuit 22 in operation 90 to return to network node “K” (event 234) tocontinue resolving the remaining links of the network node K.

Hence, the SPF link has been resolved in network node K, no outgoinglinks need to be resolved, causing the analysis of the link M→K=“?-K” atoperation 74. Each of the operations 76, 78, are skipped by theprocessor circuit 22 because the link M→K is not outgoing, and theincoming link is already marked unresolved to K “?-K”. Hence, theprocessor circuit recursively resolves the endpoint M of the link K→M inoperation 90 (event 236).

The network node M is determined by the processor circuit 22 inoperation 72 to be a safe node because it has two non-congruent paths tothe root, namely the path “?-K” via the link M→K, and the path “?-J” viathe link M→J. Starting with the shortest path link M→K, in operation 78the link is identified in operation 80 as ending in another junction“K”, enabling the link M→K to be marked by the processor circuit 22 asresolved in the state table 54 (and a first end of the collapsed arc“ARC1” of FIG. 6D and FIG. 6I). A recursive call by the processorcircuit 22 from network node M to network node K in operation 90 (event238) results in no changes, hence the processor circuit 22 returns tonetwork node M (event 240), and the processor circuit 22 resolves thenext outgoing and unresolved link M→J=“?-J” into a resolved link inoperation 80 (and the second end of the collapsed arc “ARC1” of FIG.6I).

Referring to FIG. 4B, the recursive resolution of network node “M” (asthe endpoint of the link M-K) is complete, causing the processor circuit22 return to the network node “K” at event 242 of FIG. 4C; the recursiveresolution of the network node “K” also is complete, causing the returnto network node “J” at event 244.

Note that the network nodes “K” and “M” are junction nodes withoutchanging the link reversibility. Hence, the link M→J terminates arouting arc because it ends in a junction node “J”, and the link M→Kterminates in a routing arc because it ends in another junction node“K”. Hence, the network nodes “J”, “K”, and “M” form a collapsed arc“ARC1” 12, illustrated in FIG. 6D.

The processor circuit 22 in operation 74 repeats the link analysis forthe next link of the junction node “J”, namely the outgoing linkJ→F=“?-A”. After skipping operation 76, operation 80 is skipped by theprocessor circuit 22 because the network node “F” is not a junction inFIG. 6D, hence link J→F does not end in another junction. The networknode “J” is not an intermediate junction yet because it does not havetwo reversible paths, hence operation 82 is executed to make the linkJ→F reversible (J→F=“Rev”) because the reversal of link J→F enables theendpoint network node “F” to become a junction having an alternate pathto the root network node “R” via “?-A” and the path via the network node“J”. The network node “J” was already an edge junction, hence operation84 is skipped.

Marking the link J→F reversible in operation 78, however, now makes thesafe node “J” an intermediate junction, hence operation 86 is executedby the processor circuit 22: the description in operation 86 of “pruneoutwards all links of safe node N but the 2 reversible links . . . ”ensures that there are no outward links except along the arc (formed vianetwork nodes F, J, K, and B) 12, such that any other links are prunedand directed inwards (as executed previously with respect to the linkM□→J). Operation 88 is skipped by the processor circuit 22 because thelink J→F is not incoming, and operation 90 is executed to recursivelyresolve the neighbor for the endpoint node of the link J→F, namely thenetwork node “F” (event 246).

The processor circuit 22 skips operation 70 during resolution of thenetwork node “F” it has multiple links. The network node “F” isidentified as an edge junction (and therefore a safe node) because ithas two non-congruent paths to the root node “R”, and the network node“F” includes one reversible link J→F, hence, execution of operation 72is skipped. As will be described in further detail below, the junctionnode “F” belongs to the subDAG(A) 64 a of FIG. 6B, and the junctionnodes “J”, “K”, and “B” belong to the subDAG(B) 64 b; hence, a firstinfrastructure arc (“ARC2” illustrated in FIG. 6E) 12 can be createdthat comprises (at least two or more) junction nodes and (one or more)reversible links, where one end of the first infrastructure arcterminates at the destination network node “R”.

Each of the links of the safe node “F” are analyzed in operation 74,starting with the SPF link F→C: operations 76 and 80 are skipped, andoperation 82 is executed by the processor circuit 22 to mark the linkF→C as reversible (F→C=“Rev”). Operation 84 is executed by the processorcircuit 22 because the network node “F” is now an edge junction towards“S” (S=A). Hence, in operation 84 any outgoing unresolved links labeled“?-A” (e.g., F→E=“?-A”) are pruned and reversed inwards toward the edgejunction and marked by the processor circuit 22 as unresolved toward theedge junction (e.g., change from F→E=“?-A” to E□→F=“?-F”) in the statetable 54; further, in operation 84 all incoming links (i.e., inwardlinks) of the safe node “F” are changed by the processor circuit 22 from“?-A” to “?-F” (e.g., change H→F=“?-A” and I→F=“?-A” to H→F=“?-F” andI→F=“?-F”). The relabeling of the links E□→F, H→F, and I→F in operation84 in the state table 54 exposes the network nodes “E”, “H”, and “I” tothe alternate path to the root node “R” via the edge junction “F”,enabling the network nodes “E”, “H”, and “I” to be later identified asnew junction nodes (and therefore safe network nodes) having newnon-congruent paths for reaching the root node “R”, distinct from thepath provided by the subDAG (A) 64 a. hence, the network nodes “E”, “H”,and “I” can later be used to create secondary infrastructure arcs basedon the non-congruent paths distinct from the subDAG (A) 64 a.

The edge junction “F” is not an intermediate junction yet, henceoperation 86 is skipped, and operation 88 is skipped because theincoming link E□→F is already marked “?-F” as unresolved toward the edgejunction “F”.

A recursive call is executed by the processor circuit 22 in operation 90for the endpoint network node “C” of the SPF link F→C=“Rev” (event 248).

The recursive resolution by the processor circuit 22 of the network node“C” skips operation 70 because it has multiple links. The network node“C” is identified as an edge junction (and therefore a safe node)because it has two paths to the root node “R”, and the network node “C”includes one reversible link F→C, hence, execution of operation 72 isskipped by the processor circuit 22. Operations 76 and 80 are skipped,and operation 82 is executed by the processor circuit 22 to mark thelink C→A as reversible (C→A=“Rev”) in the state table 54. Operation 84is executed by the processor circuit 22 because the network node “C” isnow an edge junction towards “S” (S=A). Hence, in operation 84 anyoutgoing unresolved links labeled “?-A” (e.g., C→D=“?-A”) are pruned andreversed inwards by the processor circuit 22 toward the edge junction“C” and marked in the state table 54 as unresolved toward the edgejunction (e.g., change from C→D=“?-A” to D□→C=“?-C”); further, inoperation 84 any incoming links of the safe node “C” are changed by theprocessor circuit 22 from “?-A” to “?-C” (e.g., change E→C=“?-A” toE→C=“?-C”) in the state table 54. The relabeling of the links D□→C andE→C in operation 84 exposes the network node “D” to an alternate path tothe root node “R” via the edge junction “C”, enabling the network node“D” to be later identified as a junction node having two non-congruentpaths for reaching the root node “R”.

The edge junction “C” is not an intermediate junction yet, henceoperation 86 is skipped by the processor circuit 22, and the link C→A isnot incoming, hence operation 88 is skipped. A recursive call isexecuted in operation 90 for the endpoint network node “A” of the SPFlink C→A=“Rev” (event 250).

The recursive resolution by the processor circuit 22 of the network node“A” skips operation 70 because it has multiple links. The heir networknode “A” is a safe node, and is identified as an edge junction becauseit has two non-congruent paths to the root node “R”, and the networknode “A” includes one reversible link C→A, hence, execution of operation72 is skipped.

The SPF link A→R is first selected in operation 74 and identified inoperation 76 as resolved. The resolved SPF link A→R also ends in theroot “R”, and therefore terminates the arc identified by the sequence ofrecursively called neighbors A(C, F, J) extending back to theintermediate junction “J”, and extending to the edge junction “B”.

Hence, the identification of the SPF link A→R as resolved during thesuccessive recursion by the processor circuit 22 from the intermediatejunction “J” (i.e., a junction node having two reversible links),establishes the junction node “A” as the second edge junction, resultingin the first infrastructure arc “ARC2” for reaching the root node “R”,illustrated in FIG. 6E and FIG. 6I. As illustrated in FIGS. 6E, 6H, and6I, the infrastructure arc “ARC2” for reaching the root node “R”includes the junction nodes “A”, “C”, “F”, “J”, “K”, and “B” connectedby the reversible links A-C, C-F, F-J, J-K, and K-B. Hence, theinfrastructure arc “ARC2” for reaching the root node “R” can beidentified based on traversing the sequence of an heir network node(e.g., “A”) and the sequence of reversible links until reaching anotherheir network node (e.g., “B”).

The next link of junction node “A” is analyzed in operation 74, namelythe link D→A=?-A, resulting in the recursive resolution of the networknode “D” in operation 90 (event 252). The network node “D” is now ajunction node (and therefore a safe node), because it has twonon-congruent paths (via nodes A and C) for reaching the root node “R”.Hence, operations 70 and 72 are skipped, and operation 74 is executedfirst for the SPF link D→A. The link D→A is marked as resolved inoperation 80 based on terminating in the junction node A. The recursivecalling from node “D” to node “A” causes the recursive analysis for node“A” to return back to node “D”, as all other links have already beenanalyzed with respect to node “A”: such a return is referred to as a“no-op recursion”, and will be omitted in future discussion for brevity.

The analysis for node “D” continues for link D□→C in operation 74. Thelink D□→C ends in the junction node “C” and therefore is marked asresolved in operation 80, resulting in the formation of the collapsedarc “ARC3” 12 illustrated in FIG. 6I. The incoming link L→D is nextanalyzed with respect to the junction node D in operation 74, andrelabeled in operation 88 from the unresolved status of ?-A to theunresolved status ?-D (L→D=“?-D”), indicating that the nearest safe nodeis the node “D”.

As illustrated in FIG. 6F, the safe node “D” can form its ownsubordinate directed acyclic graph SubDAG(D) 64 d within the SubDAG(A)64 toward the root node “R”, such that the safe node “D” becomes theclosest safe node for the network nodes “L”, “N”, “E”, “G”, “H”, and“I”. Hence, similar to operation 72, all the unresolved incoming linksin the SubDAG of safe node “D” (e.g., links L→D, E→D, E→L, G→E, H→G,I→H, and H→E) are recursively relabeled (i.e., marked) in operation 88to “?-D” to propagate the identification of the newest safe node “D”(state 252).

The recursive analysis in operation 90 of the node “L” by the processorcircuit 22 results in a no-op recursion because the node “L” is not yeta safe node, hence the analysis returns to the node “D”.

The link E→D is next analyzed with respect to the junction node D inoperation 74, and relabeled in operation 88 by the processor circuit 22from the unresolved status of ?-A to the unresolved status ?-D(E→D=“?-D”) in the state table 54. The analysis for node E isrecursively called by the processor circuit 22 in operation 90 (event254).

The network node E is a junction node (and therefore a safe node)because it has two non-congruent paths to the root via junction nodes“D” and “E”, without changing any link reversibility. The followinglinks of junction node “E” need to be analyzed in operation 74, in thefollowing example order: E→C, E→D, E→L, E□→F, G→E, and H→E.

Hence, the SPF link E→C is marked as resolved in operation 80 because itends in the junction node “C”. The outgoing link E→D is analyzed withrespect to the network node “E” in operation 74, and is marked asresolved in operation 80 (becoming the first end of the collapsed arc“ARC8” of FIG. 6I) because it ends in the junction node “D”. Theoutgoing link E→L is next analyzed in operation 74, and since the linkE→L enables in operation 82 the endpoint node “L” to become a junction,the link E→L is marked as reversible in operation 82. The endpoint “L”is recursively analyzed in operation 90 (event 256).

Referring to FIG. 4D and event 256, the network node “L” is identifiedas an edge junction (and therefore a safe node) because it has thereversible link E→L. The link L→D is marked as resolved in operation 80because it ends in the junction node “D”, resulting in the formation ofthe second infrastructure arc “ARC4” of FIG. 6G and FIG. 6I. Since thearc “ARC4” ends in a safe node “D”, then even though all traffic fromthe arc “ARC4” could exit via network node C (i.e., network node “D”sends its traffic to network node C via the link D□→C), the network node“D” still has an alternate path via network node A. The link N→L hasalready been resolved for the leaf node N, hence the analysis returns tonetwork node “E”.

The next link under analysis by the processor circuit 22 with respect tothe network node “E” (event 258) is the link E□→F=?-F in operation 74.The link E□→F is resolved in operation 80 as ending in the junction node“F” (resulting in the formation of the collapsed arc “ARC8”). Althoughthe link E□→F was pruned as unidirectional, it could be safely reversedfor LFA analysis, if desired (operation 90 is a no-op for the endpointnode F of link E□→F, hence, analysis returns to the network node “E”).

The incoming link G→E of network node “E” is next analyzed in operation74. Since the network node “G” is not a junction, it is not a safe nodeand therefore the link G→E cannot be resolved, but is relabeled ?-E inoperation 88: all incoming links to the safe node “E” also arerecursively marked by the processor circuit 22 as unresolved toward “E”(namely, links G→E, H→E, H→G, and I→H all are reset to “?-E”) resultingin the formation of a subDAG(E) toward E. Analysis of the network node“G” is recursively called as the endpoint of the link G→E in operation88.

The network node “G” (event 260) is determined to not be a junctionnode, and all links are already labeled to the nearest safe node “E”,hence operation 72 can be skipped and the processor circuit 22 canreturn back to node “E” in event 262.

The next incoming link H→E of the safe node “E” is analyzed in operation74, causing the processor circuit to recursively analyze in operation 90the endpoint node “H” at event 264.

The network node “H” in operations 72 and 74 is identified as a junctionnode having non-congruent paths via unresolved paths “?-F” (via the SPFlink H→F) and “?-E” (via the links H→E and H→G). Hence, each of thelinks of the safe node “H” are successively resolved in operation 74,namely the links H→F, H→E, H→G, and I→H.

The SPF link H→F of the safe network node “H” is resolved by theprocessor circuit in operation 80 as ending in the junction node “F”: aswill be shown later, the link H→F will terminate the infrastructure arc“ARC5” and the collapsed arc “ARC6” of FIG. 6I. Operation 90 results ina no-op recursive analysis of node “F” (as the endpoint of the linkH→F), hence, the analysis of the next (outgoing) link H→E for the safenode “H” in operation 74 causes the link H→E (ending in the junctionnode “E”) to be resolved in operation 80 as the second end of thecollapsed arc “ARC6”.

Operation 90 results in the no-op recursive analysis of node “E” (as theendpoint of the link H→E), hence the analysis of the next (outgoinglink) H→G for the safe node “H” is executed in operation 74. Inoperation 82 the link H→G enables the endpoint node “G” to become ajunction; further, the link H→G if reversed does not cause pruning ofthe SPF link H→F; hence, the link H→G is relabeled in operation 82 bythe processor circuit 22 to a reversible link (H→G=“Rev”) in the statetable 54. Operation 90 is executed for recursive analysis of theendpoint node “G” of the link H→G (event 266).

The network node “G” is determined in operation 72 to be an edgejunction (and therefore a safe node) based on the reversible link H→G.Hence, analysis of the SPF link G→E in operation 74 results in operation80 with the link G→E being labeled as resolved as the second end of theinfrastructure arc “ARC5”. Operation 90 results in the no-op recursiveanalysis of node “E” (as the endpoint of the link G→E), and since thesafe network node “G” has no other links to resolve, execution returnsto node “H” for evaluation of the next incoming link I→H (event 268).

The next link in operation 74, namely the incoming link I→H of the safenetwork node “H”: The link I→H is relabeled in operation 88 fromI→H=“?-A” to I→H=“?-H”, and the operation 90 is executed by theprocessor circuit 22 for recursive analysis of the endpoint node “I” ofthe link I→H (event 270).

The network node “I” is determined by the processor circuit 22 inoperation 72 to be a junction node having non-congruent paths viaunresolved paths “?-F” (via the SPF link I→F) and “?-H” (via theoutgoing link I→H). Hence, in operation 74 the SPF link I→F is analyzedby the processor circuit 22, and marked in operation 80 as resolved (andterminating the collapsed arc “ARC7”) based on ending in the junctionnode “F”. Operation 90 results in the no-op recursive analysis of node“F” (as the endpoint of the link I→F), resulting in analysis inoperation 74 of the next (outgoing link) I→H. Since the link I→H ends inthe junction node “H”, the link I→H is labeled in operation 114 asresolved, forming the second end of the collapsed arc “ARC7” of FIG. 6I.Operation 90 results in the no-op recursive analysis of node “H” (as theendpoint of the link I→H), returning execution analysis to junction node“I”.

Analysis of node “I” is completed by the processor circuit 22, returningexecution analysis by the processor circuit 22 to node “H”; analysis ofnode “H” is complete, returning execution analysis to node “E”; analysisof node “E” is complete, returning execution analysis to node “D”;analysis of node “D” is complete, returning execution analysis to node“A”; analysis of node “A” is complete, returning execution analysis tonode “C”; analysis of node “C” is complete, returning execution analysisto node “F”; and analysis of node “F” is complete, returning executionanalysis to node “J”. As described previously, the processor circuit 22can update the topology table 56 of FIG. 5 as each routing arc isconstructed, where the ownership of an arc cursor is identified by thejunction node having the two outwardly oriented reversible links.

The resulting link topology is illustrated in FIG. 6H, with theresulting arcs “ARC1” through “ARC8” illustrated in FIG. 6I. The routingtopology of FIG. 6I illustrates the routing arcs “ARC1” through “ARC8”,with all the network nodes being junction nodes except for the rootnetwork node “R” and the leaf node “N”. As illustrated in FIGS. 6H and6I, the collapsed arc “ARC1” includes the junction nodes “J”, “M”, and“K”; the infrastructure arc “ARC2” for reaching the root node “R”includes the junction nodes “A”, “C”, “F”, “J”, “K”, and “B” connectedby the reversible links C→A, F→C, J→F, J→K, and K→B; the collapsed arc“ARC3” includes the junction nodes “A”, “D”, and “C”; the infrastructurearc “ARC4” includes the junction nodes “D”, “L”, “E”, and “C”; theinfrastructure arc “ARC5” includes the junction nodes “E”, “G”, “H”, and“F”; the collapsed arc “ARC6” includes the junction nodes “E”, “H”, and“F”; the collapsed arc “ARC7” includes the junction nodes “H”, “I”, and“F”; and the collapsed arc “ARC8” has the junction nodes “D”, “E”, and“F”.

Consequently, assuming the link F→C encountered a failure, the networknode “F” could redirect traffic to the node “J” via the reversible linkJ→F (e.g., based on the network nodes “F” and “J” negotiating that thelink J→F needs to be reversed to F→J, enabling network traffic to beredirected without recalculation of routes.

As apparent from this disclosure, the loop-free routing topology 10 forthe destination network node (“R”) 14 can be repeated for each networknode 16, enabling each network node 16 to have its own loop-free routingtopology 10 that guarantees reachability to the corresponding networknode 16 via non-congruent paths.

Distributed Establishment of Loop-Free Label Switched Paths in theLoop-Free Routing Topology

As described previously, the loop-free routing topology 10 illustratedin FIG. 6I enables network traffic to be redirected instantaneously inresponse to a detected failure in the routing topology based onreversing a reversible link, without introducing any loops into thetopology. The loop-free routing topology 10 also can utilize a new labeldistribution protocol that enables the network nodes 16 to establishloop-free label switched paths for reaching the destination network node14 via the loop-free routing topology 10. The apparatus 20 can beconfigured for not only computing the arc topology 10, but alsogenerating a set of serialized representations describing the loop-freerouting topology, where each serialized representation describes acorresponding path in the topology: as described herein, the “path” asused herein is not necessarily limited to the disclosed routing arcs.The set of serialized representations can be propagated from thedestination network node 14 to the network nodes 16 in the computingnetwork, enabling each of the network notes to establish their ownloop-free label switched paths for reaching the destination network node14.

The apparatus can be implemented, for example, as a centralized pathcomputing engine associated with a network management system, thedestination network node 14, or any node computing the topology 10 for anumber of destinations within a prescribed autonomous system.

FIG. 7 illustrates an example hierarchy 10′ of successively cascadedrouting arcs, constructed by the apparatus 20 of FIG. 2 according to anexample embodiment. In particular, the loop-free routing topology 10 canbe represented by the apparatus 20 as a hierarchy 10′ that contains thesame routing arcs 12 for reaching the destination 14, except that therouting arcs 12 are redrawn as a hierarchy of successively cascaded(collapsed) routing arcs 12 or 12′ that supply network traffic in the“downward” direction 100 to a destination 14.

As illustrated in FIG. 7, all network traffic toward the destination 14follows the direction of the network traffic flow 100, ending at theroot 14 or the network nodes “A”, “C”, or “F” along the root arc “ARC2”12. Hence, all network traffic flows along the path 100 down thehierarchy 10′ of successively cascaded routing arcs 12 or 12′ supplyingnetwork traffic to the destination 14.

Conversely, topology control messages 102 can be propagated from thedestination network node 14 to each of the network nodes 16 in thecomputing network. The topology control messages 102 include a “set ofserialized representations” (described below) describing relevant paths(e.g., routing arcs 12) of the loop-free routing topology 10. Thetopology control message 102 is used to flood the serializedrepresentations of the relevant paths over the loop-free routingtopology 102, across each of the network nodes 16 along each of therouting arcs 12: in other words, a network node (e.g., “C”) 16 passesthe topology control message (containing the set of serializedrepresentations) to any of its neighbors that can send network trafficback to that network node (e.g., “C”), except that the network node(e.g., “C”) will not send the topology control message back to thetransmitting network node (e.g., “A”) that had just sent the topologycontrol message to the network node (“C”). Hence, each network node 16can learn the relevant paths of the loop-free routing topology 10 inresponse to parsing the set of serialized representations contained in atopology control message 102.

Each topology control message 102 also includes one or more locallysignificant labels (“λ”) generated by the network node 16 transmittingthe topology control message 102. Each locally significant labelgenerated by the transmitting network node 16 can have an arbitrarynumeric value. As described below, each locally significant label isassociated with prescribed attributes set by the transmitting networknode 16 for forwarding a data packet to the destination network node 14:as described below with respect to FIG. 15 the transmitting network node16 stores the locally significant table (and associated attributes) in alabel forwarding table (also referred to as a label switched forwardingtable); the network node receiving the topology control message 102 alsostores the received locally significant label in a new label forwardingtable entry in its own label forwarding table, creates a new locallysignificant label as an index to the new label forwarding table entry,and retransmits the set of serialized representations with the newlocally significant label in a retransmitted topology control message.

Hence, the topology control messages 102 enable the network nodes 16 toeach independently establish loop-free label switched paths for reachingthe destination network node 14 via the loop-free routing topology 10.Moreover, multiple locally significant labels can be specified within asingle topology control message 102, for example a primary label for adefault path to reach the destination, and a “backup” (or “fastreroute”) path to reach the destination. Topology control messages 102also can be propagated along both ends of a bidirectional routing arc12, resulting in a total of four locally significant labels identifyingfour respective paths available for reaching the destination node 14 bya network node 16. The loop-free label switched paths can beimplemented, for example, using multiprotocol label switched (MPLS)labels according to RFC 3031 or RFC 6178, label distribution protocol(LDP) according to RFC 3036 or 5036; alternately the labels can useother tagging techniques, for example IEEE 802.1q (or Q in Q) as labelsin carrier Ethernet, IPv6 flow labels, or direct encapsulation over ATMor frame relay. Other topology information can be transmitted over therouting arcs 12 once established, for example as illustrated in U.S.Pat. No. 7,693,064.

Hence, the label switched paths enable any network node 16 along thebidirectional paths to instantaneously reverse the transmission of adata packet to an alternate (backup) label switched path in response tothe network node detecting a failed link; moreover, loops are avoided byusing different label switched paths to identify a default path in onedirection of the bidirectional path (e.g., routing arc), a default pathin a second direction of the bidirectional path, a backup (fast reroute)path that is used in response to detecting a failed link in the defaultpath in the one direction, and a second backup (fast reroute) path thanthat is used in response to detecting a failed link in the default pathin the second direction.

FIG. 8 illustrates an example method for label distribution and routeinstallation in the loop-free routing topology, according to an exampleembodiment. Referring to FIGS. 2, 6I, and 8, the processor circuit 22 inthe apparatus 20 is configured for creating in a computing network aloop-free routing topology 10 for reaching the destination network node“R” in operation 110 as illustrated in FIG. 6I and as describedpreviously with respect to FIGS. 1 through 6I. The computing network maybe an autonomous system, or any part thereof, for example a local areanetwork, an ad hoc network such as a mobile ad hoc network, a sensornetwork, etc. As described previously, the loop-free routing topology 10comprises distinct paths 12 for reaching the destination network node14.

The processor circuit 22 also is configured for generating in operation12 a set of serialized representations describing the loop-free routingtopology 10. As described in further detail below with respect FIGS. 10and 14, each serialized representation 114 describes a corresponding oneof the paths 12. The processor circuit 22 also assembles all of theserialized representations 114 of the respective paths (e.g., routingarcs 12) into a set 116 of serialized representations, illustrated inFIG. 12. Hence, the set of serialized representations 116 can provide arepresentation of the entire loop-free routing topology 10.

The processor circuit 22 can generate in operation 118 a topologycontrol message 102 containing the set of serialized representations116, enabling the network interface circuit 24 of the destination node“R” to output in operation 118 the topology control message 102 on eachof its incoming links (i.e., toward nodes “A” and “B”) causing thetopology control message 102 to be propagated throughout the network.Since the destination node “R” 14 is the final destination for theloop-free topology 10, the destination node 14 also is referred togenerically as the “omega node” (Ω). Hence, the omega node (Ω=“R”) 14defines the forwarding equivalence class (FEC) for the topology controlmessage 102: the topology control message 102 also specifies a locallysignificant label (e.g., an MPLS label) (e.g., “O=R_AR” for the link“R-A” or “R_BR” for the link “R-B”) that is associated with the networkinterface that outputs the message to the next hop network node 16 forthe forwarding equivalence class “Ω=R”.

As used herein, the nomenclature “O=X_YZ” refers to a locallysignificant label “O” identifying the link in the direction from node“Z” to node “Y” for the forwarding equivalence class “Ω=X” to be used asa default (i.e., primary) path in reaching the destination node “Ω=X”:in other words, the node “Z” generates the locally significant label“O=X_YZ” to notify the node “Y” (i.e., topology control message “to Yfrom Z”) that the label “O=X_YZ” is to be used for forwarding a datapacket via node “Z” along a default path to the destination node “Ω=X”(i.e., data traffic destined for “Ω=X” via default path is sent “from Yto Z”). An additional extension (e.g., “FRR”) can be used to identify anadditional label attribute, namely that the link is to be used as a fastreroute (“FRR”) path (i.e., backup path) in the event of a link failureon the default path. The locally significant label can be implemented asa numeric value that provides an index into a label forwarding tablewithin a network node. Hence, a topology control message 102 output by anetwork node 16 and containing the set of serialized representations canfurther include a default label “O=X_YZ” and a backup (fast reroute)label “O=X_ZY_FRR” for the forwarding equivalence class “Ω=X”. Since therouting arcs 12 are bidirectional, the node “Y” also can receive anothertopology control message 12 from the “other end” of its arc, and inresponse send to the node “Z” a topology control message 102 specifyingthe default label “O=X_ZY”, the backup label “O=X_YZ_FRR”, and the setof serialized representations.

Hence, the omega node (Ω=“R”) 14 outputs in operation 118 (118 a of FIG.12) the topology control message 102 with the set of serialized arcs(116 of FIG. 12): the omega node (Ω=“R”) 14 also adds a locallysignificant label “O=R_AR” to the topology control message 102 outputonto the link “R-A” to the network node “A”, and a locally significantlabel “O=R_BR” to the topology control message 102 output onto the link“R-B” to the network node “B”.

In response to the network node “A” 16 receiving the topology controlmessage 102 with the locally significant label “O=R_AR” on an identifiednetwork interface (e.g., “A1”), the network node “A” can create inoperation 120 an entry in its internal label forwarding table for theforwarding equivalence class Ω=“R” that any data packet destined for thedestination node “Ω=R” 14 via the link “R-A” should be output onto thenetwork interface “A1” with the locally significant label “O=R_AR” (asdescribed previously, a locally significant label can be an arbitrarynumeric value chosen by the source of the label).

The processor circuit 22 in the network node “A” 16 also can determinethe routing topology 10 from the set of serialized arcs 116 in thetopology control message 102. Hence, the processor circuit 22 in thenetwork node “A” 16 can create additional label forwarding table entriesfor the network nodes “C” and “D” that are configured to send networktraffic to the destination node “Ω=R” 14 via the network node “A”: thelabel forwarding table entry for the network node “C” can specify a newlocally significant label “O=R_CA” (used as an index into the labelforwarding table), the destination label “O=R_AR”, and the outboundnetwork interface A1. Hence, if the network node “A” receives a datapacket from the network node “C” that specifies the locally significantlabel “O=R_CA”, the network node “A” can use the specified locallysignificant label as an index into the label forwarding table to recoverfrom the forwarding table entry the destination label “O=R_AR” (to beswapped with the existing label in the received data packet) and outputthe data packet onto the network interface “A1” for transfer to thedestination node 14 via the link “A-R”.

Hence, in operation 118 the network node “A” sends the topology controlmessage 102 to the network node “C” with the locally significant label“O=R_CA”, and to the network node “D” with the locally significant label“O=R_DA”. The network node “C” can identify the network topology 10 fromthe set of serialized arcs 116, and in response can update its labelforwarding table in operation 120 with a forwarding table entryspecifying the network interface (e.g., “C1”) having received thetopology control message 102 from the network node “A”, the locallysignificant label “O=R_CA”, and new locally significant labels (e.g.,“O=R_FC”, “O=R_EC”) used as indices into the label forwarding table fordata packets received from other network nodes (e.g., “F” via C-F; “E”via C-E). The network node “C” can output the topology control message102 to the network nodes “E” and “F” using the locally significantlabels “O=R_EC” and “O=R_FC”, respectively.

Hence, the propagation in operation 118 of the topology control messageenables the network nodes 16 2 establish the arc topology for reachingthe omega node 14; further, insertion and swapping of locallysignificant labels at each network node enables each of the networknodes 16 to independently establish loop-free label switched paths forreaching the destination network node 14 via the loop-free routingtopology 10.

FIG. 9 illustrates an example method by the processor circuit 22 of theapparatus 20 of executing operation 112 of FIG. 8, namely theserializing of the arc topology 10, according to an example embodiment.The method begins in operation 112 a based on the processor circuit 22of the apparatus 20 identifying in operation 112 b each path (e.g.,routing arc) 12 by its arc cursor 18, namely the identity of the networknode 16 having position of the arc cursor 18 for a given routing arc 12.As illustrated in FIG. 10, each routing arc 12 as identified by its arcreference (ARC1 to ARC8) in FIG. 6I is given a serialized arc identifier122 based on the corresponding network node 16 having possession of thearc cursor 18. For example, the network node “J” has possession of thecursor 18 for the routing arc “ARC2” 12; the network node “E” haspossession of the arc cursor 18 for the routing arcs “ARC4” 12 and“ARC8”, hence the serialized arc identifiers “E1” and “E2” are used todistinguish routing arcs 12 in instances where the same network node(“E”) has possession of multiple arc cursors 18 for respective routingarcs 12 (see also the serialized arc identifiers “H1” and “H2” 122 todistinguish the routing arcs “ARC5” and “ARC6” having their respectivearc cursors 18 controlled by the network node “H”).

Hence, the serialized arc identifier 122 serves as metadata thatidentifies the network node 16 in the corresponding path (e.g., routingarc) 12 as possessing the corresponding our cursor 18 for control oftraffic along the routing arc 12.

The processor circuit 20 of the apparatus 20 generates in operation 112c a list 124 (illustrated in FIGS. 10 and 14) identifying a contiguoussequence of arc nodes 16 along the corresponding arc 12. For example,the serialized arc 114 a provides the source route “{B, K, J, F, C, A}”along the arc having the serialized arc identifier “J” 122 for therouting arc “ARC2” in FIG. 6I. the processor circuit 20 also identifiesin operation 112 d at least a first edge junction and a second edgejunction of the routing arc 12 (i.e., exit nodes) that empties trafficfrom the routing arc to either the destination node 14 or anotherrouting arc that is closer to the destination network node 14 than thecorresponding routing arc 12. As used in this description, any node (ornodes) within brackets “[ ]” identify an end of a routing arc (i.e., anexit node), such that the nodes “C” and “D” are the exit nodes for therouting arc “ARC4” in the serialized arc format 114 b; the contiguoussequence of arc nodes within brackets “{ }” refers to intermediatejunction nodes that are not exit nodes.

As illustrated in FIG. 14, a serialized arc format 114 also can begenerated in operation 112 e for a buttressing arc having zero (i.e.,null) exit nodes on one end of the routing arc 12 a contiguous sequenceof arc nodes along the buttressing arc (A, B, C, and D), with a list ofexit nodes (e, f) coupled to the last network node “D”.

Hence, the serialized arc format 114 for each routing arc 12 includes afirst list of exit nodes 126, followed by a source route 124 of nodesalong the arc, ending with another list 126 of exit nodes of the arc,enabling the formation of a complete source route along thecorresponding arc 12; in the case of a buttressing arc as in FIG. 14,one of the lists 126 can have null entries, however the other list 126must specify at least one exit node for the arc. Hence, the serializedarc format 114 includes metadata for identifying the art cursor (via theserialized arc identifier 122), exit nodes (126), and the intermediatenodes 124 between the exit nodes and along the routing arc 12.

Hence, in operation 112 of FIGS. 8 and 9 the processor circuit 22 of theapparatus 20 assembles the serialized arc formats 114 for each of theindividual arcs 12 into a single set 116 of serialized arcs. Asdescribed previously, the single set 116 of serialized arcs are outputby the destination (omega) node 14 to the network nodes for propagationin operation 118 of the topology control messages 102 to enable buildingof the label switched paths by the network nodes 16.

FIG. 11 illustrates example propagation 118 of the topology controlmessages 102 throughout the network nodes 16, according to an exampleembodiment. The omega node 14 outputs in operation 118 a the topologycontrol message 102, for example as a label distribution protocol (LDP)protocol data unit (PDU). As described previously, the topology controlmessage 102 includes a set 116 of all the serialized arcs (114 a through114 h); further, each topology control message 102 output by eachnetwork node 16 specifies a corresponding unique locally significantlabel “O” for associating the LDP PDU to the forwarding equivalenceclass “Ω=R”.

In response to a network node 16 receiving in operation 118 b a topologycontrol message 102, the corresponding processor circuit 22 of thenetwork node 16 can parse from the topology control message 102 thelocally significant label “O”, and the set 116 of serializedrepresentations containing the serialized arc identifiers 122 and theserialized arc formats 114 describing the respective paths 12. Thecorresponding processor circuit 22 of the network node 16 can decode inoperation 118 c the serialized arcs, as identified by their respectiveserialized arc identifiers 122 and their respective serialized arcformats 114, and create the necessary entries into label forwardingtable of the network node 16. The network node 16 can optionally remove(i.e. proven) in operation 118 d at least one of the serializedrepresentations from the received topology control message 102 based onthe corresponding path being positioned between the network node and thedestination 14. The pruning of operation 118 d is described in furtherdetail below with respect to FIG. 12. The network node 16 outputs inoperation 118 e a modified (i.e., pruned) topology control message 102on each of its inward arcs of the arcs topology 10, for propagation tothe next network nodes in the topology 10.

FIG. 12 illustrates a selective pruning of selected paths 12 from theset 116 of serialized representations propagated in the topology controlmessage 102 by network nodes within the loop-free routing topology 10,according to an example embodiment. The specific serialized arc formats114 are omitted from FIG. 12 for simplicity, hence each arc isrepresented in FIG. 12 merely by its serialized arc identifier 122.

Operations 128 a through 128 m illustrate operation 118 d of FIG. 11executed by the different outputting network nodes. As illustrated withrespect to operation 128 a, there is no pruning performed in the outputset of serialized arcs 116 when transmitting the topology controlmessage 102 from the destination node (Ω=R) 14 to the nodes along theroot arc “ARC2”, namely from the destination node (Ω=R) 14 to eithernode “A” or “B”, from node “A” to “C”, from node “C” to node “F”, fromnode “F” to node “J”, etc. to node “B”; from node “B” to node “K”, fromnode “K” to node “J”, etc.

Hence, the topology control message 102 output from the destination node(Ω=R) 14 to node “A” is propagated around the arc “ARC2” 12, enablingeach network node 16 node along the path ARC2″ 12 in the first directionto update its label forwarding table with the source node's locallysignificant label O, and swap with a new locally significant label O′for use by the next hop node; similarly, the topology control message102 output from the destination node (Ω=R) 14 to node “B” is propagatedaround the arc “ARC2” 12, enabling each network node 16 along the path“ARC2” 12 in the second direction to update its label forwarding tablewith the source node's locally significant label O″, and swap with a newlocally significant label O′″ for use by the next hop node.

In contrast, the network nodes 16 in the routing arcs above the root arc“ARC2” 12 (identified by its serialized art identifier “J” 122) do notneed the topology information for the root arc; hence, the network nodes“A”, “C”, and “F” can selectively prune the full set of serialized arcs116 a as illustrated in operations 128 b, 128 c, 128 d, 128 e. forexample, in operation 128 b the network nodes “A” and “C” can prune theserialized arcs “J” and “M” (as represented in FIG. 10) from the set ofserialized arcs 116 a, in order to send the pruned set 116 b to thenetwork node “D”; similarly, the network nodes “C” and “E” can prune inoperation 128 c the serialized arcs “J”, “M”, and “D” from the set ofserialized arcs 116 a, in order to send the pruned set 116 c to thenetwork node “E”; the network node “F” can prune in operation 128 d theserialized arcs “J”, “M”, “D”, “E1”, and “E2” from the set of serializedarcs 116 a, in order to send the pruned set 116 d to the network node“H”; the network node “F” also can prune in operation 128 e theserialized arcs “J”, “M”, “D”, “E1”, “E2”, “H1”, and “H2” from the setof serialized arcs 116 a, in order to send the pruned set 116 e to thenetwork node “I”.

Operations 128 f through 128 l illustrated further pruning that can beexecuted by other nodes in arcs that are above the root arc ARC2. Hence,each network node can selectively prune at least one of the serializedrepresentations 114 from the received topology control message 102 basedon the corresponding path 12 being positioned between the at least onenetwork node and the destination network node 14, and output themodified (i.e., pruned) topology control message to another one of thenetwork nodes away from the destination and the selectively pruned path.

FIG. 13 illustrates an example method of establishing the loop-freelabel switched paths by the network nodes receiving the set when 16 ofserialized representations from the destination network node 14,according to an example embodiment.

As described previously, the sourcing network node (e.g., thedestination node (Ω=R) 14 or an exit node of a lower routing arc 12)outputs a topology control message 102 specifying the forwardingequivalence class (FEC) for the destination node (Ω=R) 14; the topologycontrol message 102 also specifies a locally significant label O that isunique for the sourcing node based on path direction, whether the pathis the primary (default) path or a fast reroute (backup) path, arcidentifier, and the forwarding equivalency class.

Assume the receiving network node that receives in operation 130 thetopology control message 102 is node “F” that receives the topologycontrol message from the node C: the topology control message specifiesthe label “O=R_FC” generated by the node “C” (i.e., for the FEC=R,output on link C-F in the “LEFT” direction from node F to node C). Inresponse, the node “F” decodes in operation 132 the set of serializedarcs for the forwarding equivalence class and installs an entry in thelabel forwarding table associated with the locally significant label“O=R_FC” specified in the topology control message 102.

FIG. 15 illustrates an example label forwarding table 148 stored in thememory circuit 26 of the network node “F” 16, according to an exampleembodiment. In particular, the processor circuit 22 of the node “F”creates an entry in operation 134 of FIGS. 13 and 15, using the newlabel “O=R_JF” created by node “F” as an index: hence, any data packetreceived by node “F” (e.g., from node J) containing the label “O=R_JF”is swapped with the left primary swap label (LP) “O=R_FC” and output onthe network interface identified as “InterfaceLP_ID” to the link F-C forswitching to the node “C”; if the node “F” detects a failure on the linkC-F, then fast reroute can be instantly implemented by swapping thelabel “O=R_JF” with the right-direction fast reroute (RFF) label“O=R_FJ_FRR” and outputting the packet on the interface“InterfaceRFF_ID” for transmission of the data packet via the link F-Jback to the node “J”.

The network node (e.g., “F”) can choose in operation 136 a new locallysignificant label for each of the other next hop nodes that did not sendthe topology control message 102, and forward the topology controlmessage on the inward links (e.g., F-J, F-I, F-H, or F-E). The relevanttable entries also are created in operation 150 of FIG. 15. For example,the network node “F” would send to the network node “J” the topologycontrol message 102 specifying the set of serialized representations,and the labels “O=R_JF” for the default path and “O=R_FJ_FRR” for thefast reroute (FRR) path.

Assuming in operation 138 that another topology control message 102 isreceived from the other side of the arc (e.g., “F” receives the message102 from node “J” with the label “O=R_FJ”), the node “F” can add thecorresponding entry in the label forwarding table in operation 140,including a right primary swap label (RP) and a left fast reroute (LFF),enabling association of the locally significant label (e.g., “O=R_CF”)as described previously, but in the opposite direction.

The processor circuit 22 in each network node 16 also implementsswitching rules in operation 142, illustrated in FIG. 15 based on theincoming label. In particular, for Inter-arc switching 152, either theleft primary or right primary labels may be used depending on theposition of the network node relative to the arc cursor. For intra-arc(i.e. within the arc) switching 154, loop-free fast reroute is utilizedin response to a detected local link failure: (1) a left primary (LP)label is swapped with a Right Fast Reroute (RFF) label; (2) a rightprimary (RP) label is swapped with a Left Fast Reroute (LFF) label; and(3) any received packet having a RFF label or LFF label is dropped toprevent the formation of a loop (156 of FIG. 15).

Hence, fast reroute is implemented instantly in response to a detectedlink failure: four label switched paths are maintained by each networknode, namely: left-primary (in the left direction), right-fast reroute(in the right direction in response to a detected link failure forleft-primary), right-primary (in the right direction), and left-fastreroute (in the left direction in response to a detected link failurefor right-primary). Further, a fast reroute label state is propagated inthe rerouted direction; hence, a second link failure in the same arcwill cause the data packet to be dropped, avoiding the formation of aloop.

The topology control message received in operation 144 also is forwardedafter a new locally significant label is chosen.

Hence, labels can be established within a routing topology that enablesfull redundancy in two connected graphs: a new label distributionprotocol format is introduced that contains a new non-linear sourceroute information to describe the routing arc topology. Each routing arcallows one link failure, hence the arc topology can support multipleconcurrent link failures that might occur in different routing arcs.Further, the label distribution protocol can be deployed automaticallyand independently by the network nodes in the computing network.

Creating Non-Congruent Paths in the Loop-Free Routing Topology HavingRouting Arcs

FIGS. 16A-16C illustrate creation of non-congruent paths 300 a and 300 b(illustrated in FIGS. 16B and 16C) in the loop-free routing topology 10that enables bicasting of network traffic between a source network nodeand a destination network node 14, according to an example embodiment.The term “bicasting” refers to a network node within the loop-freerouting topology 10 (e.g., a host network node such as a controllerdevice, a sensor device, a user device, etc.) that outputs two datapackets containing the same payload for delivery to a destination: onedata packet is routed to a destination via a first multi-hop networkpath to the destination, and a bicasted copy of the data packet isrouted to the destination via a second multi-hop network path that isdistinct from the first multi-hop network path. Bicasting is effectivein deployments where reliability and timeliness must be guaranteed.Bicasting also can be used in video distribution.

As an example of bicasting, a single packet could be routed via therouting arcs 12 along the shortest path to the destination network node(i.e., “root”) 14, based on the above-described routing of traffic awayfrom the arc cursor 18; however any such routing of a data packet mayrequire utilizing fast reroute to reverse the flow for the data packetin response to a failure in the network topology.

According to an example embodiment, the non-congruent paths 300 a and300 b guarantee that no single point of failure within the loop-freerouting topology 10 will cause a disruption in any network traffic thatis concurrently transmitted (e.g., bicasting) via the two or morenon-congruent paths. In particular, bicasted data packets are routed viathe non-congruent paths 300 a and 300 b and independent of the arccursors 18, to guarantee that even with a breakage in the networktopology two non-congruent paths can be maintained.

FIG. 16A illustrates the loop-free routing topology 10 comprising therouting arcs “A1”, “A2”, “A3”, “A4”, “A5”, “A6”, “A7”, and “A8” 12, andthe buttressing arcs “BA1”, “BA2”, and “BA3” 13, prior to assignment ofany routing arc to any non-congruent path: any unassigned routing arc 12or buttressing arc 13 is illustrated in FIGS. 16A-16C as a solid line.FIG. 16B illustrates initial formation of the non-congruent path 300 awithin the routing arcs “A1” and “A2” and the buttressing arc “BA2”,where each non-congruent path 300 a is illustrated as a dashed line;FIG. 16B also illustrates initial formation of the non-congruent path300 b within the routing arcs “A1”, “A2” and “A3”, where eachnon-congruent path 300 b is illustrated as a long-dashed andshort-dashed line (with alternating long-dash and short-dash). FIG. 16Cillustrates the completed formation of the non-congruent paths 300 a and300 b within the routing topology 10.

FIGS. 14 and 16A-16C each illustrate a routing arc 12 joined with asingle buttressing arc 13. However, any number of buttressing arcs 13may be joined with a routing arc 12 at the same junction node or adifferent junction node, enabling the formation of a comb structurecomprising a single routing arc and multiple buttressing arcs joined tothe routing arc.

As described in further detail below, each bicasted data packet includesa tag that specifies or identifies the direction that the data packet isto be transmitted, namely along either the non-congruent path 300 a or300 b: if a data packet includes a tag specifying the “West/Left” path300 a, the data packet is forwarded along the non-congruent path 300 a,whereas the bicasted copy of the data packet having the tag specifyingthe “East/Right” path 300 b will be forwarded along the non-congruentpath 300 b. As described below, a data packet also can be forwarded awayfrom the non-congruent path (e.g. 300 a) toward the arc cursor 18 if thecorresponding tag specifies the opposite direction (e.g., 300 b).Consequently, the bicasting data packets are routed to the non-congruentpaths 300 a and 300 b based on the directions as specified by the tags,respectively.

Hence, a data packet does not need to be reversed (using fast rerouting)in response to a detected failure within a first path (e.g., 300 a) ofthe loop-free routing topology, as the second non-congruent path (e.g.,300 b) can provide the concurrently-transmitted packet between thesource and destination. As described below, fast rerouting can be usedin response to detected collisions either during formation of thenon-congruent paths, or during establishment of non-congruent multicastpaths for transmitting multicast streams from the multicast source(i.e., the root) 14 to one or more network nodes in the loop-freerouting topology 10 that have subscribed to the multicast streams.

FIG. 17 illustrates an example method of creating non-congruent paths300 a and 300 b in the loop-free routing topology 10, according to anexample embodiment. FIG. 18 illustrates example multicast registrationby network nodes using the non-congruent paths 300 a and 300 b in theloop-free routing topology 10, and multicasting via non-congruentmulticast paths established based on the non-congruent paths 300 a and300 b, according to an example embodiment. Each of the disclosedoperations with respect to FIG. 17 and FIG. 18 can be executed by theprocessor circuit 22 of the apparatus 20, or any logic encoded on atangible computer readable storage medium, as described previously.

The processor circuit 22 can create non-congruent paths 300 a and 300 bwithin the loop-free routing topology 10 based on execution ofoperations 302, 304, 306, 308, and 310 of FIG. 17. The processor circuit22 can begin creation of the non-congruent paths based on associatingeach heir network node (i.e., each first hop node from the destinationroot node 14) in operation 302 to one and only one non-congruent path300 a or 300 b. For example, FIG. 16B illustrates the processor circuit22 associating the heir network nodes “H1”, “H2”, and “H3” with the“West/Left” non-congruent path 300 a, and the heir network nodes “H4”,“H5”, and “H6” with the “East/Right” non-congruent path 300 b.

The processor circuit 22 in operation 304 propagates the association ofeach heir network node along the corresponding routing arc 12 toward thecorresponding network node having possession of the arc cursor 18 in arouting arc 12, or alternately toward the junction node of a buttressingarc. For example, the association of the heir network node “H2” to thenon-congruent path 300 a is propagated up along the routing arc “A2”toward the corresponding arc cursor 18, causing all network nodes thatare along the routing arc “A2” in between the heir network node “H2” andthe arc cursor 18 (e.g., network nodes “N11” and “N16”) to be associatedwith the non-congruent path 300 a. Similarly, the association of theheir network node “H5” to the non-congruent path 300 b is propagated upalong the routing arc “A2” toward the corresponding arc cursor 18,causing all network nodes that are along the routing arc “A2” in betweenthe heir network node “H5” and the arc cursor 18 (e.g., network nodes“N14” and “N13”) to be associated with the non-congruent path 300 b.Similar associations are propagated in the routing arc “A1”. Both therouting arcs “A1” and “A2” are root arcs having their ends of therouting arcs as the first hop heir network nodes, such that all of thenetwork nodes in the root arcs “A1” and “A2” have a direct associationwith one of the heir network nodes. The association of the heir networknode “H6” to the non-congruent path 300 b is propagated in operation 304up the routing arc “A3” up to the corresponding arc cursor 18. Theassociation of the heir network node “H1” to the non-congruent path 300a is propagated up the entire buttressing arc “BA2” 13 up to andincluding the edge junction node “J1” 16.

The processor circuit 22 in operation 306 performs implied associationbetween the routing arcs based on selectively propagating theassociation along a routing arc to another higher routing arc via thenetwork nodes that serve as edge junctions between the lower (parent)routing arc and the higher routing arc. In particular, edge junctionnodes (e.g., “N11” and “N12”) of a higher routing arc (e.g., “A6”)propagate the heir node association from the parent arcs if the edgejunction nodes (e.g., “N11” and “N12”) are exposed to differentnon-congruent paths. As illustrated in FIG. 16B, the edge junction node“N11” of the routing arc “A6” is exposed to the “West/Left”non-congruent path 300 a of the parent routing arc “A2”, whereas thecomplementary edge junction node “N12” of the routing arc “A6” isexposed to the “East/Right” non-congruent path 300 b of the parentrouting arc “A3”. Hence, the heir node associations from the parent arcscan be propagated along the routing arc “A6” up to the corresponding arccursor 18, where the processor circuit 22 causes the network node “N11”to propagate the association to the non-congruent path 300 a up therouting arc “A6” to the corresponding arc cursor 18, and causes thenetwork node “N12” to propagate the association to the non-congruentpath 300 b up the routing arc “A6” to the corresponding arc cursor 18.

The selective propagation of the associations along a routing arc alsoincludes the processor circuit 22 in operation 308 resolving any“collisions” if edge junction nodes (e.g., “N13”, “N14”) in a higherrouting arc (e.g., “A5”) are joined to the same non-congruent path 300 bvia the at least one lower routing arc (e.g., “A2”) providingreachability to the destination network node 14. The processor circuit22 associates the edge junction node (e.g., “N14”) having the shorterpath to an heir network node “H5” with the exposed non-congruent path300 b, and associates the second edge junction (e.g., “N13”) (that isfurther than “N14” to any heir network node) with the other(non-exposed) non-congruent path 300 a, enabling the routing arc “A5” tohave edge junctions “N13” and “N14” providing reachability to thenon-congruent paths 300 a and 300 b, respectively.

The processor circuit 22 can continue in operation 310 the propagationof associations of the non-congruent paths 300 a or 300 b upward alongthe routing arcs 12, resulting in the complete association of all therouting arcs in FIG. 16C to either the non-congruent path 300 a or thenon-congruent path 300 b.

Hence, the processor circuit 22 enables any network node in theloop-free routing topology 10 to utilize the non-congruent paths 300 aand 300 b in operation 312 for forwarding bicasting data via thenon-congruent paths 300 a and 300 b. As illustrated in FIGS. 16C and 19,a network node “N20” in the buttressing arc “BA3” 13 can output a datapacket having a tag specifying the “West/Left” path 300 a, causing thenetwork nodes in the loop-free routing topology 10 to forward the datapacket via the non-congruent path 300 a that includes the network nodes“J2” (via buttressing arc “BA3”), “N15” (via routing arc “A8”), “N11”(via routing arc “A6”), “H2” (via routing arc “A2”), to the root networknode 14. Similarly, the network node “N20” can output a bicasted copy ofthe data packet having a complementary tag specifying the “East/Right”path 300 b, causing the network nodes in the loop-free routing topology10 to forward the bicasted copy via the non-congruent path 300 b thatincludes the network nodes “N17” (via buttressing arc “BA3”), “N12” and“H6” (via routing arc “A3”), to the root network node 14. FIGS. 16C and19 also illustrate bicasting by the network node “N21” along thenon-congruent paths 300 a and 300 b.

Hence, the bicasting by the network nodes “N20” and “N21” via thenon-congruent paths 300 a and 300 b guarantee that at least one of thebicasted data packets will reach the destination root 14. If theprocessor circuit 22 determines that a particular path along one of therouting arcs encounters a failure, the processor circuit 22 can executea fast reroute onto a fast reroute bicasting path in operation 314, toestablish an alternate non-congruent path, if needed. As describedbelow, the multicast source 14 can also use the non-congruent paths 300a and 300 b as illustrated in FIG. 19 as non-congruent multicast pathsfor multicast traffic to the nodes “N20” and “N21”.

FIG. 18 illustrates the processor circuit 22 establishing in operation316 non-congruent multicast paths utilizing the non-congruent paths 300a and 300 b, according to an example embodiment. As described withrespect to operation 312 and FIG. 19, if the data packet that isbicasted contains a multicast request generated by the network node(e.g., “N20” or “N21”), each network node along the non-congruent path300 a or 300 b can mark the new multicast registration request(implemented, for example, as a protocol independent multicast (PIM)message) in a local routing table, for example by the incoming linkidentifier, direction identifier (identifying the path 300 a or 300 b),and multicast identifier. Assuming a given network node has not alreadyreceived the multicast registration request from the same direction(e.g., “West/Left”), the multicast request is forwarded along thespecified non-congruent path 300 a or 300 b. If the network node hasalready received a multicast registration request for the same direction(e.g., “West/Left”), the multicast registration request can bedisregarded as redundant in view of the multicast registration requestpreviously stored in the local routing table.

As apparent from the foregoing, multiple network nodes may initiatetheir own multicast registration requests for consumption of themulticast stream to be transmitted by the multicast source 14. Hence,instances may arise where a single network node (e.g., “N13”) mayreceive in operation 318 another multicast request for the samemulticast stream, but for a different direction (e.g., “East/Right”):this “collision” can occur if multicast requests are crossing the samerouting arc 12 via different entry points from higher routing arcs; this“collision” also can occur if multicast requests from different sets ofhigher routing arcs exit into the same junction node of a parent routingarc. In both cases, the processor circuit 22 in operation 320 can causethe network node (e.g., “N13”) to detect that the second bicasted copyof the multicast registration requests that specifies the conflictingdirection “East/Right”, and in response cause the network node (e.g.,“N13”) to reject the bicasted copy that caused the collision by sendinga reject message back and initiating a fast reroute for the conflictingdirection (e.g., “East_FRR”) to the upstream network node that causedthe collision (e.g., “N18”); the processor circuit 22 also can cause thenetwork node (e.g., “N13”) to prune the incoming “East/Right” path fromthe multicast stream availability in order to remove the overlappingpath. Further, the processor circuit 22 can cause the network node(e.g., “N13”) to relabel any multicast streams from the root 14 havingthe conflicting “East/Right” label to be re-labeled as the registerednon-congruent path 300 a, for retransmissions upward to the registerednodes via the registered non-congruent path 300 a. Hence, the networknode (e.g., “N13”) maintains the non-congruent path only for the firstregistered direction 300 a, rejects any requests in the conflictingdirection 300 b, and relabels any media streams specifying theconflicting direction 300 b to the registered direction 300 a toguarantee non-congruency of the multicast path. The network node thatinitially transmitted the conflicting multicast registration request(e.g., “N18”) can respond to the reject message specifying the fastreroute direction (e.g., “East_FRR”) by establishing an alternatenon-congruent path for reaching the destination 14, and pruning the linkproviding the reject message to prevent any further multicast conflictsin the conflicting direction.

The following examples can further illustrate the collision mediation ofmultiple multicast registration request from different directions.Referring to FIG. 20A, assume in a first example that the network node“N22” receives a first multicast registration request “R1” (for the“East/Right” direction) from the routing arc “A10” 12, marks the firstmulticast registration request “R1” in its local (internal) routingtable, and forwards the first multicast registration request “R1” alongthe routing arc “A9” toward the network node “N23” for exiting theright-hand side exit node of the routing arc A9 (operations 316 and 318for network node “N22”). Assume further the network node “N23” receivesa second multicast registration request “R2” from the routing arc “A11”for the same multicast stream as the first multicast registrationrequest, except for the second multicast registration request specifyingthe “West/Left” direction: the network node “N23” marks the secondmulticast registration request “R2” in its local (internal) routingtable, and forwards the second multicast registration request “R2” tothe network node “N22”.

In response to the network node “N22” receiving in operation 318 thesecond multicast registration request “R2” from the network node “N23”,the network node “N22” detects the collision based on detecting thesecond multicast registration request “R2” specifies the “West/Left”direction in conflict with the “East/Right” direction marked for thepreviously-received first multicast registration request “R1” from therouting arc “A10” 12. In response, the network node “N22” forwards thesecond multicast registration request “R2” along the arc “A9” to thenetwork node “N24” and marks the second multicast registration request“R2”: as described below, the marking of the second registration request“R2” enables the network node “N22” to identify that relabeling of amulticast stream from “West/Left” to “East/Right” may be needed. Thenetwork node “N2” also can execute collision resolution based on sendinga path unavailability message to the network node “N23”, specifying thepath from network node “N23” to “N22” is not available for servicing thesecond multicast registration request “R2” for the “West/Left”direction. The network node “N22” also can perform collision mediationby pruning the incoming path (from node “N23” along arc A9) frommulticast availability to remove the overlapping path for the specifiedmulticast stream.

The network node “N23” can receive, mark, and forward the firstmulticast registration request “R1” along the routing arc “A9” to theexit network node “N25: the marking of “R1” and “R2” having conflictingdirections enables the network node “N23” to detect the collision, andin response prune the path between nodes “N22” and “N23” along therouting arc “A9”, and perform any relabeling of multicast streams,described below. The network node “N23” also can respond to the pathunavailability message indicating unavailability of the path between“N23” and “N22” by pruning the path (from node “N23” along arc A9 tonode “N22”) from multicast availability for the specified multicaststream. Hence, the network node “N23” can perform collision resolutioneither based on the path availability message, and/or based on detectingthe conflicting directions between the requests “R1” and “R2”, dependingon the order in which the messages “R1”, “R2”, and/or the pathunavailability message are received by the network node “N23”.

Hence, the network node “N22” receives the multicast stream (labeled“West/Left”) via node “N24” for propagation to the arc “A10” afterrelabeling as “East/Right”; and the network node “N23” receives themulticast stream (labeled “East/Right”) via the network node “N25” forpropagation to the arc “A11” after relabeling as “West/Left”. Hence, thenetwork nodes “N22” and “N23” perform collision mediation to guaranteethe non-congruency of the multicast streams, based on pruning theoverlapping path along the routing arc “A9” between the network nodes“N22” and “N23”.

FIG. 20B illustrate another collision mediation example, where multicastregistration packets for different directions from routing arcs “A13”and “A14” are received by the network node “N26”. Assume the networknode “N26” receives from the network node “N31” (from routing arc “A15”)via the routing arc “A13” a first multicast registration request “R1”specifying a first direction (e.g., “East/Right”), causing the networknode “N26” to mark and forward the request “R1” toward the network node“N27” along arc “A12” for delivery to the destination 14 via the networknode “N29”. In response to the network node “N26” receiving a secondmulticast registration request “R2” specifying a conflicting direction(e.g., “West/Left”) from the network node “N32” via arc A14 (receivedfrom arc A16), the network node “N26” can execute collision mediation bysending a reject message of the second multicast registration request“R2” back to the node “N32”, and the network node “N26” pruning the pathbetween “N26” and “N32” via “A14” for the “West/Left” direction. Thenetwork node “N32” forwards the second multicast registration request“R2” (specifying “West/Left”) using its fast reroute path along the arc“A14” to the exit node “N33”. If the exit node “N33” cannot forward thesecond multicast request to the exit node “N30” along arc “A17” usingthe “West/Left” direction label, the exit node “N33” can “recolor”(i.e., relabel) the direction to another direction that is differentthan the original “West/Left” direction label, and forward the secondmulticast request to the network node “N30”. Collision mediation can berepeated, as needed, by downstream network nodes until the multicastregistration requests are received by the destination 14.

Hence, the network nodes of FIG. 20B can establish a first multicaststream from the root 14 upwards via the network nodes “N29”, “N27”,“N26”, and “N31”, and a second non-congruent multicast stream via thenetwork nodes “N30”, “N33”, and “N32”. Relabeling (recoloring) can beperformed at any nodes to resolve any rerouting that was required forcollision avoidance.

Hence, the multicast source 14 can send the multicast streams along thenon-congruent multicast paths in operation 322 based on the collisionresolution implemented in the network nodes 16 forwarding theregistration requests to the multicast source 14. Hence, the collisionresolution guarantees the non-congruency of the multicast paths.

According to example embodiments, non-congruent paths are establishedfor bicasting data within a computing network having a loop-free routingtopology. The non-congruent paths also can be utilized for establishingnon-congruent multicast paths based on forwarding the multicast requestsalong the non-congruent paths, and applying collision resolution toguarantee no single point of failure can adversely affect reception ofthe multicast streams by any network node in the loop-free routingtopology. Further, fast reroute can be employed for instantaneouscollision resolution with respect to any single node receiving requestsutilizing conflicting paths.

While the example embodiments in the present disclosure have beendescribed in connection with what is presently considered to be the bestmode for carrying out the subject matter specified in the appendedclaims, it is to be understood that the example embodiments are onlyillustrative, and are not to restrict the subject matter specified inthe appended claims.

What is claimed is:
 1. A method comprising: creating, in a computingnetwork, a loop-free routing topology comprising a plurality ofsuccessively cascaded routing arcs for reaching a destination networknode, the successively cascaded routing arcs including a higher routingarc configured for emptying network traffic originated by at least onesource network node into one or more lower routing arcs, each routingarc comprising a first network node as a first end of the routing arc, asecond network node as a second end of the routing arc, and at least athird network node in between the first end and the second end andconfigured for routing any network traffic along the routing arc towardthe destination network node, the first and second ends available forenabling exiting of any of the network traffic from the correspondingrouting arc toward the destination network node, the loop-free routingtopology providing first and second non-congruent paths via one or moreof the successively cascaded routing arcs, wherein the correspondingthird network node of each of the higher routing arc and the one or morelower routing arcs is distinct from the source network node; andforwarding bicasting data, comprising a data packet in a first directionfrom a network node and a bicasted copy of the data packet in a seconddirection from the network node, concurrently to the destination networknode respectively via the first and second non-congruent paths.
 2. Themethod of claim 1, wherein the creating includes: associating first-hopnodes, that are one hop from the destination network node, as belongingto one of the first non-congruent path or the second non-congruent path;and propagating the association of each first-hop node along thecorresponding routing arc toward one of the corresponding third networknode, or a junction node of a buttressing arc.
 3. The method of claim 2,wherein at least one of the routing arcs is a root arc having thecorresponding first network node as the first-hop node belonging to thefirst non-congruent path and the corresponding second network node asthe corresponding first-hop node belonging to the second non-congruentpath, the third network node of the at least one routing arc controllingthe network traffic along the corresponding at least one routing arc viathe respective first or second non-congruent paths of the correspondingrouting arc.
 4. The method of claim 2, wherein the creating furtherincludes selectively propagating the association along the correspondingrouting arc to a second routing arc via the corresponding first networknode serving as an edge junction node between the second routing arc andthe corresponding routing arc, for propagation along the second routingarc of the association to the one non-congruent path.
 5. The method ofclaim 4, wherein the selectively propagating includes: identifying thatthe first network node and the second network node of the second routingarc are joined to the first non-congruent path via at least one lowerrouting arc providing reachability to the destination network node;associating the first network node of the second routing arc with thefirst non-congruent path based on determining the first network node hasa shorter path to the destination network node via the firstnon-congruent path than the corresponding second network node via thefirst non-congruent path; associating the second network node of thesecond routing arc with the second non-congruent path based on thecorresponding first network node associating with the firstnon-congruent path; propagating along the second routing arc therespective associations of the first and second network nodes.
 6. Themethod of claim 1, wherein the forwarding includes: forwarding the datapacket in the first direction along the first non-congruent path via oneof the routing arcs, in response to detecting a tag in the data packetidentifying the first direction; and forwarding the bicasted copy ofdata packet away from the first direction along the first non-congruentpath and toward the second non-congruent path, in response to detectingthe tag in the bicasted copy of the data packet identifying the seconddirection.
 7. The method of claim 1, further comprising sendingmulticast streams from the destination network node to the network node,via first and second non-congruent multicast paths, in response to thedestination network node receiving the data packet and the bicasted copyeach containing a multicast registration request, each network nodealong one of the first or second non-congruent multicast paths markingthe multicast registration request in response to reception thereof. 8.An apparatus comprising: a processor circuit configured for creating, ina computing network, a loop-free routing topology comprising a pluralityof successively cascaded routing arcs for reaching a destination networknode, the successively cascaded routing arcs including a higher routingarc configured for emptying network traffic originated by at least onesource network node into one or more lower routing arcs, each routingarc comprising a first network node as a first end of the routing arc, asecond network node as a second end of the routing arc, and at least athird network node in between the first end and the second end andconfigured for routing any network traffic along the routing arc towardthe destination network node, the first and second ends available forenabling exiting of any of the network traffic from the correspondingrouting arc toward the destination network node, the loop-free routingtopology providing first and second non-congruent paths via one or moreof the successively cascaded routing arcs, wherein the correspondingthird network node of each of the higher routing arc and the one or morelower routing arcs is distinct from the source network node; and anetwork interface circuit configured for causing forwarding of bicastingdata, comprising a data packet in a first direction from a network nodeand a bicasted copy of the data packet in a second direction from thenetwork node, concurrently to the destination network node respectivelyvia the first and second non-congruent paths.
 9. The apparatus of claim8, wherein the processor circuit is configured for creating the firstand second non-congruent paths based on: associating first-hop nodes,that are one hop from the destination network node, as belonging to oneof the first non-congruent path or the second non-congruent path; andcausing propagating of the association of each first-hop node along thecorresponding routing arc toward one of the corresponding third networknode, or a junction node of a buttressing arc.
 10. The apparatus ofclaim 9, wherein at least one of the routing arcs is a root arc havingthe corresponding first network node as the first-hop node belonging tothe first non-congruent path and the corresponding second network nodeas the corresponding first-hop node belonging to the secondnon-congruent path, the third network node of the at least one routingarc controlling the network traffic along the corresponding at least onerouting arc via the respective first or second non-congruent paths ofthe corresponding routing arc.
 11. The apparatus of claim 9, wherein theprocessor circuit further is configured for selectively propagating theassociation along the corresponding routing arc to a second routing arcvia the corresponding first network node serving as an edge junctionnode between the second routing arc and the corresponding routing arc,for propagation along the second routing arc of the association to theone non-congruent path.
 12. The apparatus of claim 11, wherein theprocessor circuit is configured for: identifying that the first networknode and the second network node of the second routing arc are joined tothe first non-congruent path via at least one lower routing arcproviding reachability to the destination network node; associating thefirst network node of the second routing arc with the firstnon-congruent path based on determining the first network node has ashorter path to the destination network node via the first non-congruentpath than the corresponding second network node via the firstnon-congruent path; associating the second network node of the secondrouting arc with the second non-congruent path based on thecorresponding first network node associating with the firstnon-congruent path; propagating along the second routing arc therespective associations of the first and second network nodes.
 13. Theapparatus of claim 8, wherein the processor circuit is configured forcausing: forwarding the data packet in the first direction along thefirst non-congruent path via one of the routing arcs, in response todetection of a tag in the data packet identifying the first direction;and forwarding the bicasted copy of data packet away from the firstdirection along the first non-congruent path and toward the secondnon-congruent path, in response to detecting the tag in the bicastedcopy of the data packet identifying the second direction.
 14. Theapparatus of claim 8, wherein the processor circuit is configured forcausing multicast streams from the destination network node to be sentto the network node, via first and second non-congruent multicast paths,in response to the destination network node receiving the data packetand the bicasted copy each containing a multicast registration request,each network node along one of the first or second non-congruentmulticast paths marking the multicast registration request in responseto reception thereof.
 15. Logic encoded in one or more non-transitorytangible media for execution and when executed by a machine operablefor: creating, in a computing network, a loop-free routing topologycomprising a plurality of successively cascaded routing arcs forreaching a destination network node, the successively cascaded routingarcs including a higher routing arc configured for emptying networktraffic originated by at least one source network node into one or morelower routing arcs, each routing arc comprising a first network node asa first end of the routing arc, a second network node as a second end ofthe routing arc, and at least a third network node in between the firstend and the second end and configured for routing any network trafficalong the routing arc toward the destination network node, the first andsecond ends available for enabling exiting of any of the network trafficfrom the corresponding routing arc toward the destination network node,the loop-free routing topology providing first and second non-congruentpaths via one or more of the successively cascaded routing arcs, whereinthe corresponding third network node of each of the higher routing arcand the one or more lower routing arcs is distinct from the sourcenetwork node; and forwarding bicasting data, comprising a data packet ina first direction from a network node and a bicasted copy of the datapacket in a second direction from the network node, concurrently to thedestination network node respectively via the first and secondnon-congruent paths.
 16. The logic of claim 15, wherein the creatingincludes: associating first-hop nodes, that are one hop from thedestination network node, as belonging to one of the first non-congruentpath or the second non-congruent path; and propagating the associationof each first-hop node along the corresponding routing arc toward one ofthe corresponding third network node, or a junction node of abuttressing arc; wherein at least one of the routing arcs is a root archaving the corresponding first network node as the first-hop nodebelonging to the first non-congruent path and the corresponding secondnetwork node as the corresponding first-hop node belonging to the secondnon-congruent path, the third network node of the at least one routingarc controlling the network traffic along the corresponding at least onerouting arc via the respective first or second non-congruent paths ofthe corresponding routing arc.
 17. The logic of claim 15, wherein thecreating includes: associating first-hop nodes, that are one hop fromthe destination network node, as belonging to one of the firstnon-congruent path or the second non-congruent path; propagating theassociation of each first-hop node along the corresponding routing arctoward one of the corresponding third network node, or a junction nodeof a buttressing arc; and selectively propagating the association alongthe corresponding routing arc to a second routing arc via thecorresponding first network node serving as an edge junction nodebetween the second routing arc and the corresponding routing arc, forpropagation along the second routing arc of the association to the onenon-congruent path.
 18. The method of claim 1, wherein at least one ofthe routing arcs is a root arc having the corresponding first networknode as a first-hop node belonging to the first non-congruent path andbeing one hop from the destination network node, the root arc furtherhaving the corresponding second network node as the correspondingfirst-hop node belonging to the second non-congruent path and being onehop from the destination network node.
 19. The apparatus of claim 8,wherein at least one of the routing arcs is a root arc having thecorresponding first network node as a first-hop node belonging to thefirst non-congruent path and being one hop from the destination networknode, the root arc further having the corresponding second network nodeas the corresponding first-hop node belonging to the secondnon-congruent path and being one hop from the destination network node.20. The logic of claim 15, wherein at least one of the routing arcs is aroot arc having the corresponding first network node as a first-hop nodebelonging to the first non-congruent path and being one hop from thedestination network node, the root arc further having the correspondingsecond network node as the corresponding first-hop node belonging to thesecond non-congruent path and being one hop from the destination networknode.